debug: separate DEBUG and QEMU build configs, add DEBUG linker script

[jthornblad]

Description

• Split QEMU and DEBUG into distinct build modes for tkey_app and tkey_uecc: QEMU keeps -O0/-g3/-mno-relax for debugging under QEMU, while DEBUG now builds with -Oz/-mrelax/-flto and lld's --icf=safe/--relax-gp for a smaller, relaxed release-like debug build (was previously sharing QEMU's unoptimized flags)
• Add tkey_debug.ld linker script (128 KB RAM) for the new DEBUG target, separate from tkey_qemu.ld (1 MB) and the default tkey.ld
• Add tkey_uecc_qemu.mk / tkey_uecc_qemu.a as its own uECC build target for QEMU, decoupled from tkey_uecc_debug.mk
• Rename tkey_uecc_debug.mk's define from QEMU_DEBUG to TKEY_DEBUG to match its new DEBUG-only role
• Minor cleanup: drop stale commented-out flags, fix indentation in tkey_qemu.ld and tkey_uecc.mk

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)
• Feature (non breaking change which adds functionality)
• Breaking Change (a change which would cause existing functionality to not work as expected)
• Documentation (a change to documentation)

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

[dehanj]

Maybe make comment that tkey-libs also should be built with -mrelax