v0.2.0 — nRF54LM20A support

Adds nRF54LM20A support and the Seeed XIAO nRF54LM20A board (xiao_nrf54lm20a).

SoftDevice selection is now per board. nRF54LM20A uses S145 v10.0.1 (the first
S145 release with an nRF54LM20A build, from sdk-nrf-bm v2.0.1); nRF54L05/L10/L15
keep the vendored v9.0.0 unchanged and build identically to v0.1.0.

Not verified on hardware. The XIAO nRF54LM20A pin mapping comes from the
upstream Zephyr board definition and has not been checked against a real board.
Published from a fork branch so the bootloader hex can be consumed by
platform-nordicnrf54 for testing.