feat(build_library): put all of the partition stuff behind a flag

this lays the groundwork for supporting a cpio img which doesn't have
partitions.
feat(build_library): pxe support

This adds support for creating a PXE image and kernel using build_image
fix(build_library): fixup naming for the pxe image

based on @marineam's comments fixup the naming and ensure the output
files get picked up by the build system.
Merge pull request #94 from philips/pxe-initial

Pxe initial