fix(pxe_update_engine): Sync the mini update engine with recent changes.
add(systemd): Import current units and reboot_manager from overlay.
rename(pxe): Rename pxe services to a more generic "stub" name.

This makes the stub applicable to other diskless images we may add such
as bootable ISOs. Instead of relying on kernel command line options
switching between the real update_engine and the stub is now determined
by a /usr/.noupdate flag file we can include in our images.
fix(update_engine_stub): Set default OEM_ID to "diskless"

Since writing this we've started booting via ISOs so PXE is no longer
the only boot scenario that will trigger this stub.
Merge pull request #23 from marineam/pxe

PXE related updates