download/coreboot: clone upstream via ./gitclone

coreboot trees/patching is still handled
specifically by "./download coreboot"

command now available in lbmk:

./gitclone coreboot

this *only* creates the directory at:
coreboot/coreboot

this directory is never used in builds.
it is only used by download/coreboot to
create patched trees for each mainboard