v0.2.3 — nRF54LM20A boots standalone

The nRF54LM20A bootloader now boots from its own hex with no hand-patching.

Fixed: nothing occupied address 0, so a flashed board locked up at reset.
nRF54L has no MBR — Nordic puts the reset-vector owner at 0x00000000 — while
this port inherited the nRF52 layout that expects an MBR there. src/boot_stub.S
adds a reset vector inside the 4 KB page already reserved below the
application, and points VTOR at the bootloader before jumping.

Fixed: dropped the nRF52-legacy UICR sections at 0x10001014/0x10001018.
Those addresses are unmapped on nRF54LM20A (its UICR is at 0x00FFD000), so the
hex carried a segment the part does not have and flashers rejected or skipped it.

Verified on a XIAO nRF54LM20A: resets into VTOR=0x001D0000, no lockup, UARTE20
live on P1.11/P1.10, parked in bootloader_dfu_start() awaiting a serial DFU host.

nRF54L05/L10/L15 are unchanged and still have the same address-0 gap.