OpenLegacyBoot: Add grub4dos (grldr) PBR detection support

[bugprogrammer]

Summary

Adds grub4dos (grldr) PBR detection to OpenLegacyBoot, allowing OC to
chain-load grub4dos as a standalone legacy boot entry.

Background

OpenLegacyBoot currently detects NTLDR, BOOTMGR, and ISOLINUX PBR
signatures but not grldr. grub4dos is a legacy (MBR) boot loader
chain-loaded via OpenLegacyBoot to boot NT5 systems (Windows 2000/XP).

Unlike Vista/7, which can boot natively via UEFI+GPT (requiring CSM only
for video), NT5 requires a full legacy BIOS environment and cannot boot
through UEFI at all. In OpenDuet or CSM-based UEFI systems, ntldr's
strict DL=0x80 requirement often fails because the firmware's disk
enumeration differs from native BIOS. grub4dos serves as an intermediary
that performs the necessary drive mapping (map () (hd0)) before
chain-loading ntldr.

With grldr detection, each NT5 installation can have its own grub4dos
PBR and appear as an independent entry in the OC picker, rather than
relying on a single shared grub4dos menu with multiple entries.

Changes

• LegacyBootInternal.h: Add OcLegacyOsTypeGrldr enum value.
• LegacyBootSupport.c: Detect both "GRLDR" and "grldr" PBR
  signatures. Case-insensitive search is required because FAT32 stores
  directory entries in uppercase while NTFS preserves the original
  lowercase filename.
• OpenLegacyBoot.c: Add Grub4dos (legacy) entry name. Flavour reuses
  OC_FLAVOUR_WINDOWS (consistent with all existing legacy entries), so
  no new theme assets are required.

The existing NTLDR detection branch is preserved unchanged — partitions
without grub4dos installed continue to appear as Windows (legacy).

Testing

• EP45 (OpenDuet) + Windows 2000: OC detects grldr PBR, chain-loads
  grub4dos, boots Win2000 successfully.
• Z77 (UEFI + CSM) + Windows XP: Same flow, boots successfully.
• Native BIOS boot (without OC): grub4dos menu.lst with conditional
  map works correctly.

Recommended menu.lst

For environments where the target disk may not be hd0 (e.g. OpenDuet
disk enumeration), a conditional map avoids errors on native BIOS boot:

find --set-root /ntldr
if not "%@root%"=="(hd0,0)" map () (hd0) && map --hook
chainloader /ntldr

[vit9696]

The change simply changes the name of the boot entry from what I understand. I.e. previously grub4dos would be detected as Windows (legacy) now it would be called Grub4dos (legacy). Does the boot entry title really matter given it is still essentially Windows, although booted via grub?

[bugprogrammer]

The change simply changes the name of the boot entry from what I understand. I.e. previously grub4dos would be detected as Windows (legacy) now it would be called Grub4dos (legacy). Does the boot entry title really matter given it is still essentially Windows, although booted via grub?

This is not just a title change. The core of this PR is adding grldr (grub4dos) PBR signature detection to InternalDetectLegacyOsType in LegacyBootSupport.c.

Previously, OpenLegacyBoot only recognized three PBR signatures:

if (CheckLegacySignature ("BOOTMGR", Buffer, BufferSize)) {
    LegacyOsType = OcLegacyOsTypeWindowsBootmgr;
} else if (CheckLegacySignature ("NTLDR", Buffer, BufferSize)) {
    LegacyOsType = OcLegacyOsTypeWindowsNtldr;
} else if (  CheckLegacySignature ("ISOLINUX", Buffer, BufferSize)
          || CheckLegacySignature ("isolinux", Buffer, BufferSize))
{
    LegacyOsType = OcLegacyOsTypeIsoLinux;
} else {
    LegacyOsType = OcLegacyOsTypeNone;  // grldr fell here — partition was invisible
}

A partition with grub4dos installed to its PBR contains the grldr signature, which did not match any of the above, so LegacyOsType was OcLegacyOsTypeNone and the partition never appeared in the OC boot menu.

This PR adds a new detection branch:

} else if (  CheckLegacySignature ("GRLDR", Buffer, BufferSize)
          || CheckLegacySignature ("grldr", Buffer, BufferSize))
{
    LegacyOsType = OcLegacyOsTypeGrldr;
}

The boot entry title ("Grub4dos (legacy)") is adapted accordingly as a consequence, not the purpose of the PR.

The use case is booting Windows 2000/XP (NT5): OpenLegacyBoot's direct ntldr boot has a DL register mapping issue on non-0x80 disks (ntldr requires the boot disk to be 0x80). grub4dos's map command solves this. With each NT5 system having its own grub4dos PBR, OC can list and boot them independently, rather than funneling everything through one grub4dos menu.

[vit9696]

Right, might you want it to be detected as Windows for more native icons though? If not, I could merge as is.

[bugprogrammer]

Right, might you want it to be detected as Windows for more native icons though? If not, I could merge as is.

Please merge as-is. Keeping the title as "Grub4dos (legacy)" is intentional during the setup phase: grub4dos requires configuring menu.lst (disk mapping, chainloading targets, etc.), and being able to distinguish the grub4dos entry from native ntldr entries in the picker is essential for debugging.

Once configured, the entry name can be customized per-partition using OpenCore's built-in disk label mechanism, which already works with legacy boot entries:

1. Enable the disk label attribute in config.plist:

Misc -> PickerAttributes -> OC_ATTR_USE_DISK_LABEL_FILE (0x80)

2. Load OpenNTFS.efi driver so OC can read files from NTFS partitions.
3. Place a .disk_label.contentDetails file at the root of the target partition containing the desired display name (e.g. Windows XP).

OpenLegacyBoot already checks for this file via LoadAppleDiskLabel() and uses its content to override the default entry name. No code changes are needed for renaming — the default "Grub4dos (legacy)" title serves as a sensible fallback when no custom label is present.