softdevice needs the memory at 0x2000 to be initialized to 0xffffffff
according to #5893 and testing. However, the addresses [0x8bc, 0x3000[ are not
set in softdevice.hex.
So use a modified hex file with all the memory set to 0xff as it is the rom
reset value anyway.
This change updates the `.hex` file instead on relying on erasing the
memory.
When running tests using 'nordic_softdevice_ble', the 'softdevice.hex'
file must also be taken into account for the test hashing and be
uploaded to the separated murdock testing boards.
When listed as dependency from `test-input-hash` the file must have a
target, which he has not. The file is implicitly created when compiling
`ELFFILE` so declare it as order only dependency.
In practice `BASELIBS` could be enough or even `pkg-build-softdevice`
but I do not want to be depend that much on internals there.
