build/clean: Add helper script to clean U-Boot builds

Copy the resources/scripts/build/clean/crossgcc script and adapt it to
run "make distclean" on U-Boot build trees. Some build artifacts persist
after the run, so also run "git clean -fdx" if we can.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>