to my knowledge, this feature has never been used,
but lbmk permits resources/coreboot/boardname/extra.sh
to execute, as provided by the maintainer, with working
directory set to: coreboot/boardname

this could be used to extend lbmk in a number of ways

for example, it could be used to patch 3rdparty/

it could also be used to break coreboot in creative
and novel ways. hint hint.