Disk builder support for arbitrary files

[jasoncouture]

The disk builders should support the addition of arbitrary files

This is useful for a number of reasons:

1. Smaller OSes may want a single disk, and may be ok with FAT
2. Microkernels may want to load additional services at boot
3. Bootloader file based configuration
4. Ramdisk support
5. Bootloader support for loading arbirary files

Adding all of these files to the same implementation will be cumbersome, due to duplicate of code, so I propose the following:

1. Get rid of UefiBoot and BiosBoot, and replace them with a common builder that has functions for UEFI, BIOS, and TFTP
2. Rather than treating the kernel as a special case, use a BTreeMap on the new implementation to track source/destination

The proposed changes can be found in #320