feat(build): per-board memory.x override with MCU-family fallback

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.

ld/wio_tracker_l1-memory.x

@@ -0,0 +1,25 @@
+/* Wio Tracker L1 memory layout — UF2 bootloader, NO SoftDevice
+ *
+ *   0x00000 – 0x01000   MBR (Master Boot Record)
+ *   0x01000 – 0xEA000   Application  ← we live here (no SD present)
+ *   0xEA000 – 0xF4000   Reserved (app data)
+ *   0xF4000 – 0xFC000   UF2 Bootloader
+ *   0xFF000 – 0x100000  Bootloader settings
+ *
+ * Background: Seeed factory firmware ships s140 SoftDevice at
+ * 0x01000–0x26000, but our earlier UF2 flashes (with the original
+ * `ORIGIN = 0x1000` layout) silently overwrote that region. The WIO
+ * bootloader now reports `SoftDevice: not found`; until we restore
+ * s140 via SWD, the app must live at 0x1000 to match the bootloader's
+ * no-SD boot chain.
+ *
+ * Delete this file once SD is reinstalled — `build.rs` will then fall
+ * back to `ld/nrf52840-memory.x` (the SD-aware default at 0x26000)
+ * and the WIO matches every other nRF52840 board.
+ */
+
+MEMORY
+{
+  FLASH : ORIGIN = 0x00001000, LENGTH = 0xE9000
+  RAM   : ORIGIN = 0x20000008, LENGTH = 255K
+}