readFirmware() can fail unexpectedly on the ARM9

[AntonioND]

readFirmware() on the ARM9 works by asking the ARM7 to read the firmware and save the result to a pointer passed by the ARM9. If this pointer is in DTCM (for example, if there is an array in the stack) this will fail because the ARM7 can't see DTCM.

There are two options:

1. We can document this limitation and add an sassert() to the code:

https://github.com/blocksds/libnds/blob/3e300238523fa5ca8560fdcb76dabce32d10170a/source/arm9/storage/firmware.c#L11

2. We could also use malloc() to obtain temporary storage that the ARM7 can see, then copy the data to the final destination in DTCM.