Add check in MidiBoot that it's not trying to flash a bootloader

[antisvin]

We have a check in firmware that it's flashing a bootloader - https://github.com/RebelTechnology/OpenWare/blob/master/Source/MidiHandler.cpp#L325-L327

But it looks like MidiBoot itself doesn't do the opposite and just flashes any incoming data - https://github.com/RebelTechnology/OpenWare/blob/master/Source/MidiBoot.cpp#L264-L276

This can lead to a situation when bootloader gets flashed from bootloader mode by mistake.