Support a .noinit section for variables

This is inspired by the linker sripts on the AVR architecture, which
support a .noinit section (or any section starting with .noinit,
actually) for variables that should be allocated an address in RAM, but
not be initialized to any particular value (not even zero) on startup.
These can then be used to remember values across resets.

From the sketch perspective, this works exactly the same as on AVR: Just
annote a global variable with `__attribute__((__section__(".noinit")))`
and it will have an unpredictable value on power-up and retain its value
during resets.

To implement this without having to change all board-specific linker
scripts, the linker commandline is changed to pass the board-specific
linker script to the `--default-script` linker script, and change the
main linker script (passed to `--script`, previously `-T`) to a generic
"override" linker script. This new generic linker script contains an
`INSERT BEFORE` command, which causes the linker to load it *in addition
to* the default linker script, while adding an extra `.noinit` output
section in the right place.

Because these new variables take up RAM but have their own section in
the .elf file, they should be accounted for in the size summary after
compilation. This is done by adapting the `recipe.size.regex.data` entry
to include this new section.

platform.txt

@@ -128,7 +128,7 @@ recipe.S.o.pattern="{compiler.path}{compiler.S.cmd}" {compiler.S.flags} {build.i
 recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}"
 
 ## Combine gc-sections, archives, and objects
-recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.flags} "-T{build.variant.path}/{build.ldscript}" "-Wl,-Map,{build.path}/{build.project_name}.map" {compiler.c.elf.extra_flags} {compiler.ldflags} {compiler.arm.cmsis.ldflags} -o "{build.path}/{build.project_name}.elf" "-L{build.path}" -Wl,--start-group {object_files} {compiler.libraries.ldflags} "{archive_file_path}" -lc -Wl,--end-group -lm -lgcc -lstdc++
+recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}" {compiler.c.elf.flags} "-Wl,--default-script={build.variant.path}/{build.ldscript}" "-Wl,--script={build.system.path}/ldscript.ld" "-Wl,-Map,{build.path}/{build.project_name}.map" {compiler.c.elf.extra_flags} {compiler.ldflags} {compiler.arm.cmsis.ldflags} -o "{build.path}/{build.project_name}.elf" "-L{build.path}" -Wl,--start-group {object_files} {compiler.libraries.ldflags} "{archive_file_path}" -lc -Wl,--end-group -lm -lgcc -lstdc++
 
 ## Create output (.bin file)
 recipe.objcopy.bin.pattern="{compiler.path}{compiler.objcopy.cmd}" {compiler.elf2bin.flags} {compiler.elf2bin.extra_flags} "{build.path}/{build.project_name}.elf" "{build.path}/{build.project_name}.bin"
@@ -147,7 +147,7 @@ recipe.output.save_file={build.project_name}.{build.variant}.hex
 ## Compute size
 recipe.size.pattern="{compiler.path}{compiler.size.cmd}" -A "{build.path}/{build.project_name}.elf"
 recipe.size.regex=^(?:\.text|\.data|\.rodata)\s+([0-9]+).*
-recipe.size.regex.data=^(?:\.data|\.bss)\s+([0-9]+).*
+recipe.size.regex.data=^(?:\.data|\.bss|\.noinit)\s+([0-9]+).*
 recipe.size.regex.eeprom=^(?:\.eeprom)\s+([0-9]+).*
 
 

system/ldscript.ld

@@ -0,0 +1,44 @@
+/*
+ * This script extends the default linker script to add a .noinit
+ * section. This section is just mapped to RAM, but it is emitted
+ * separately from the .data and .bss sections (both of which are
+ * initialized by startup code), so any variables in this section are
+ * untouched on startup (so they survive across resets).
+ *
+ * This script is intended to supplied to the linker's -T / --script
+ * option as the primary linker script. When the linker sees an INSERT
+ * command, this will cause it to *also* read the default linker script
+ * (after reading this script) and then executing the INSERT commands
+ * after both scripts have been read.
+ *
+ * Note that parsing of linker scripts is a bit peculiar, e.g. INSERT
+ * does not textually inserts, it inserts any generated output sections.
+ * Also, because this script is read *first*, we cannot refer to things
+ * in the default script. In particular, it would make sense to add >
+ * RAM to the output section below to ensure that the section is mapped
+ * into RAM, but the RAM region is not defined yet (I think it would
+ * work though, but produces warnings). Instead, we just rely on the
+ * defaults used by the linker: If no region is defined for an output
+ * section, it will just map to first address after the previous section
+ * (.bss in this case, which is fine).
+ */
+SECTIONS
+{
+  /* Define a noinit output section and mark it as NOLOAD to prevent
+   * putting its contents into the resulting .bin file (which is the
+   * default). */
+  .noinit (NOLOAD) :
+  {
+    /* Ensure output is aligned */
+    . = ALIGN(4);
+    /* Define a global _snoinit (and _enoinit below) symbol just in case
+     * code wants to iterate over all noinit variables for some reason */
+    _snoinit = .;
+    /* Actually import the .noinit and .noinit* import sections */
+    *(.noinit)
+    *(.noinit*)
+    . = ALIGN(4);
+    _enoinit = .;
+  }
+}
+INSERT AFTER .bss;