Enable the handling of flashing `softdevice.hex` when flashing the firmware
for openocd.
However, for flashing, only the `hexfile` and `binfile` can currently be used.
The `elffile` is generated with local pages aligned to `0x10000` which makes
the program starting at `0x1f000` be flashed from `0x10000` with padding bytes
even if the `.text` section is indeed at `0x1f000`:
readelf --sections bin/nrf52dk/gnrc_networking.elf
...
[ 1] .text PROGBITS 0001f000 00f000 00f698 00 AX 0 0 16
...
readelf --segments bin/nrf52dk/gnrc_networking.elf
...
LOAD 0x000000 0x00010000 0x00010000 0x1e6a0 0x1e6a0 R E 0x10000
...
The padding bytes would go through `verify_image` in `openocd` so be expected
to not be overwritten but are by `softdevice.hex`
Using --nmagic at link time removes the local page alignement but would
need dedicated testing.
fe0b829d5b