Small fixups for makefiles

My manifest puts libbootimg into external/.
This fixes up relative paths broken for that location.

Upstream suggests to put libbootimg into system/extras/,
but I prefer different locations.
[tmp] Don't include multirom_hooks.h

The file does not exist and is not necessary.
This is a hack for Ubuntu Touch on mako.
Might reconsider/revert, if the file gets added.
Fix broken layouts for 1280x800 devices
Rename internal ROM to primary ROM
Big ugly patch

This needs review and splitting up.
The Asus blob code should be moved to a separate library, that
can be pulled in and enabled via device config.