trees: auto-delete+auto-rebuild if project changes

actual source code is not scanned, but config directories are
scanned. simply get the checksum of each file under config/
pertaining to a given project/tree, and also for the given
target. coreboot utilities are also handled.

if it changes, in any way, delete and re-build automatically.

such deletions should probably still be done manually, as part
of understanding the build system, but this change should make
the build system much easier to use during development.

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