Automate CBFS size calculations in an helper

[tlaurion]

So maybe 0xBDEFFF is the maximum CBFS size? I will recompile, flash, and see. It would be cool if we could automate this math.

Basically, the right calculation is taking output of ifdtool -f layout.txt path/to/ifd_output_from_me_cleaner_on_fullrom_backup.rom and parsing the BIOS region, substracting end address to begin address.

@rbreslow Seems like xx30 and xx20 might have been off all along. Redoing.

Originally posted by @tlaurion in https://github.com/linuxboot/heads/pull/1282#discussion_r1089815298