Build: Don't depend on uds.hex and udi.hex

[mchack-work]

Description

synth.json shouldn't depend on uds.hex and udi.hex because that triggers a complete rebuild of the bitstream if the UDI or UDS are changed.

Instead, we want only the application_fpga.asc target to depend on them, so it can patch in the changed UDS and UDI with tools/patch_uds_udi.py in an existing application_fpga_par.json.

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Bugfix (non breaking change which resolve an issue)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes

[agren]

This looks good to me.

Doing an incremental build with updated UDI and UDS now works. The resulting application_fpga.bin is equivalent to application_fpga.bin built on main with the same UDI and UDS.