download/coreboot: re-add book-burning support

see:

https://en.wikipedia.org/wiki/Book_burning



i'll actually update blobs.list for each coreboot rev
in a subsequent commit. this logic was taken from an
old libreboot revision, which uses different coreboot
revisions. as i write this, i'm running deblob-check
from linux-libre deblob scripts.

my process is: i just check each file and decide whether
it's a blob, or like, test data. in some cases it flags
other false positives, like... a C source file that has
a bunch of magic numbers in it for things (not a blob)

Signed-off-by: Leah Rowe <leah@libreboot.org>