build.rs resolves the linker layout in two steps: per-board override
`ld/{board}-memory.x` first, MCU-family default second. Lets a board
deviate from the family default without a per-board build matrix.
Adds `ld/wio_tracker_l1-memory.x` with `ORIGIN = 0x1000` (no-SD
layout). The Wio Tracker L1 ships from Seeed with s140 SoftDevice, but
our earlier `ORIGIN = 0x1000, LENGTH = 0xE9000` linker silently
overwrote it across many UF2 flashes; the bootloader now reports
`SoftDevice: not found`. Until s140 is restored via SWD, the WIO
needs the no-SD layout to match the bootloader's actual boot chain.
Delete the per-board file once SD is restored -- the build will then
fall back to the SD-aware `ld/nrf52840-memory.x` and the WIO matches
every other nRF52840 board.
swaits
1c98786ce6