Documentation: Bring back abuild documentation

Based on contents from coreboot wiki[1], this patch adds much needed
documentation for the very important abuild utility.

On top of what was there:
- Mainboard targets have been updated
- Added example for building one variant of one board
- Added example for building boards selectively and/or with custom
  configurations using --skip_set/--skip_unset, -K, and config files

[1] https://www.coreboot.org/Abuild

Change-Id: I69701eaeef616828bc30736aba2f617e844a3148
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76775
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

util/util_readme/post_util.md

@@ -1,5 +1,6 @@
 ## In depth documentation
 
+* [abuild](util/abuild/index.md)
 * [cbfstool](util/cbfstool/index.md)
 * [ifdtool](util/ifdtool/index.md)
 * [intelp2m](util/intelp2m/index.md)