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.