nrf52: linker script lookup uses literal name, ignores PIO->Adafruit alias (nrf52_xxaa.ld)

[zackees]

Symptom

Companion to #325 — same root cause, different artifact. After the variant include path is resolved (#325), the nrf52840_dk integration test now fails at the link step with:

+ "```" +
arm-none-eabi-gcc link failed:
ld: cannot open linker script file
/home/runner/.fbuild/prod/cache/platforms/dl-framework-arduinoadafruitnrf52-1.10601.0/.../cores/nRF5/linker/nrf52_xxaa.ld:
No such file or directory
+ "```" +

Adafruit's BSP ships SoftDevice-flavored scripts only:

+ "```" +
cores/nRF5/linker/nrf52840_s140_v6.ld
cores/nRF5/linker/nrf52832_s132_v6.ld
cores/nRF5/linker/nrf52_common.ld
+ "```" +

Root cause

+ "Nrf52Cores::get_linker_script" + ( + "crates/fbuild-packages/src/library/nrf52_core.rs:98-100" + ) joins + "<linker_dir>/<ldscript_name>" + literally. Board JSONs that track PIO/sandeepmistry upstream (e.g. + "crates/fbuild-config/assets/boards/json/nrf52840_dk.json" + ) declare + "ldscript = "nrf52_xxaa.ld"" + — the bare-metal name from the sandeepmistry/arduino-nRF5 framework. Adafruit's BSP uses different names for the same hardware, gated on MCU.

Fix

Add an MCU-keyed alias resolver mirroring + "resolve_nrf52_variant_dir" + from PR #326 / #325:

input ldscript	MCU	resolved name
nrf52_xxaa.ld	nrf52840	nrf52840_s140_v6.ld
nrf52_xxaa.ld	nrf52832	nrf52832_s132_v6.ld
any	other	literal (no alias)

Literal name wins when it exists on disk so the alias is a strict fallback. Implemented in PR #326 as a follow-up commit.

Impact

Unblocks the + "tests/platform/nrf52840_dk" + integration test and FastLED CI workflow + "build_nrf52840_dk" + (FastLED/FastLED#2631) once the new release is cut and ingested.