Update README
Small fixups for makefiles

My manifest puts multirom into bootable/ and libbootimg into external/.
This fixes up relative paths broken for those locations.

Upstream suggests to put both into system/extras/, but I prefer
different locations.
Change filename of final ZIP

Put the MultiROM version before the date.
Use out-of-tree signapk.jar

Java is not in my build tree, so can't build signapk.
Generates an unsigned ZIP, if the out-of-tree signapk.jar does not exist.
Fix segfault in fb_draw_overlay()

Divide fb size by two, if using 16-bit array to access it.
Remove mt_switch_xy

The code broke touch on TF300T. Anyway, all transformations
from reported coor to screen coor should be in one place.
Rename "Internal" ROM to "Primary"

This should be less confusing, I hope.
At least I was confused initially. ;-)
Allow booting primary ROM via kexec
[tmp] Remove Ubuntu support

Will be added back, once Android ROMs are working stable.
[installer] Package kernel

Stores the current bootimage in roms/Primary/boot.img.
Uses packaged kernel instead of current one.
This will boot primary ROM using kexec.
[installer] Package modified TWRP