Improve boot devices terminology

[binarymaster]

Currently Cromwell have this terminology for boot devices:

• CD for booting system from DVD drive
• FATX for booting system from FATX (E:) data partition of Xbox
• Native for booting system from MBR formatted HDD, and from any filesystem supported by the GRUB code, which is partly incorporated into Cromwell

This issue is for discussion how we should improve the terminology for better code readability.