update docs/maintain/ for recent lbmk changes

lbmk was massively re-designed since audit2.

we're currently on audit3, though audit3 is not yet
complete, but people are using lbmk.git every day
to build ROM images.

therefore, it is important that we keep the documentation
up to date. this file documents the entire build system.

there are 13 scripts, at the time of this revision,
versus 18 as of audit2 and about 50 in the previous
release before audits 2 and 3; that, and the code is
nearly 50% reduced in size, with no loss of function!

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