Add a menu to select emulator.img before starting trezor

[markweenwork]

hi - great project - thanks for all the work put into this!

modern sd cards are much bigger than needed for this , i would like to use 1 sdcard for all my wallets .

multiple emulator.img files could be stored in a folder and possibly symlink back to the one required by selecting it in a menu with the two buttons before the trezor platform loads.

this could make managing multiple wallets much more convenient without having loads of sd cards