build system: fix alignment function

It seriously miscomputed alignment values, always
off-by-one, and off-by-an-alignment for aligned
values.

Change-Id: Ide3477d09d34d7728cb0666bb30dd9f7a3f1056d
Reported-by: Dave Frodin <dave.frodin@se-eng.com>
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7635
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Dave Frodin <dave.frodin@se-eng.com>