fix(build_image): The kernel is now in /usr/boot in /usr images
feat(disk_util): Switch from SYSLINUX's gptmbr.bin to mbr.bin

cgpt now supports generating hybrid MBRs and the classic style mbr.bin
from any version of SYSLINUX should work the same with the hybrid MBR.
The other code, gptmbr.bin, changes after SYSLINUX 3. Switching lets me
play with different versions of SYSLINUX without breaking everything.

With this change all images feature a hybrid MBR so the special case for
some VM platforms has been removed.
Merge pull request #187 from marineam/usr

fix(build_image): The kernel is now in /usr/boot
Merge pull request #188 from marineam/syslinux

feat(disk_util): Switch from SYSLINUX's gptmbr.bin to mbr.bin
fix(build_library): create /usr/share/coreos files

create the basic files in /usr/share/coreos that should exist
hack(build_library): support amd64-usr

Make special defaults for amd64-usr