implement import_xprv <xprv> and save_xprv <filename> commands
Instead of loading master private key from SD card we should be able to load it from Serial interface (kinda from command line). It will be useful for development.
Also it would be nice to be able to save it to SD card then. save_xprv command is called without a filename it should save master private key to xprv.txt file in the root of the SD card