libtrellis/tools/ecpmulti.cpp: Added the dual-boot/multi-boot jump table just before the end of the SPI flash

[trabucayre]

ecpmulti allows users to store multiple bitstreams in the same bin/mcs files. By pressing PROGRAMN, the next bitstream is loaded in a specific order.

However, the jump table is missing, meaning the golden bitstream is not automatically used if the primary bitstream is corrupted.

The code to generate the jump table already exists but was never added at flashsize & 0xff.

This PR adds the missing jump table, enabling behavior similar to Lattice's ddtcmd. It also adds two new arguments:

• --golden to pass golden bitstream file name
• --goldenaddr to pass golden bitstream address.

Notes:

• according to FPGA-TN-02203-1.8:
  • first bitstream is placed at offset 0x00,
  • second bitstream is the golden bitstream and placed at a address depending on the target FPGA
  • subsequent bitstreams are considered as alternate and placed to upper addresses.
• According to ddtcmd, only the golden bitstream address is present in the jump table.

Testing:

Successfully tested with ecp5_evn and other ECP5-based boards.

[trabucayre]

CI error is unrelated to the PR but to a missing dependency in arch-pkg: Fixed by PR #245.