mkit: as: fix signedness issue when loading map files

Adding signed header bytes to calculate sizes fails for values > 127.
Reported by ArtemioUrbina. Fixes issue #1, part 1.

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>
mkit: as: fix loading of maps in which the BODY chunk is last

Reported and fixed by ArtemioUrbina. Fixes issue #1, part 2.

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>