coreboot_tables: Make existing alignment conventions more explicit

coreboot_tables: Make existing alignment conventions more explicit

There seem to be some recurring vague concerns about the alignment of
coreboot table entries. While the existing implementation has been
producing tables with a well-defined alignment (4 bytes) for a long
time, the code doesn't always make it very clear. This patch adds an
explicit constant to codify that alignment, assertions to check it after
each entry, and adds explicit padding to the few entry structures that
were relying on compiler padding to return a correct sizeof() value.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: Iaeef29ef255047a855066469e03b5481812e5975
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70158
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Peter Stuge <peter@stuge.se>