Modified error handling

Simplified the error handler in mlb, removing the reboot on keypress
and instead just looping a hlt. Rebooting a failed bootloader probably
wouldn't help much, anyway.

Added single-letter error codes to mlb to explain what went wrong.

Moved the version check to the installer. There is no need to check this
at boot time unless the user is doing something very silly.

The above changes have netted a nice 40 b savings in the mlb binary size.

Updated the Readme to reflect the changes.