lib.sh: new function mk() to handle trees in bulk

single-tree projects cannot be handled in bulk, e.g.
./mk -f project1 project2 project3

that is still the case, from the shell, but internally
it is now possible:
mk -f project1 project2 project3

mk() is a function that simply handles the given flag,
and all projects specified.

it does not handle cases without argument, for example
you cannot do:
mk -f

arguments must be provided. it can be used internally,
to simplify cases where multiple single-tree projects
must be handled, but *also* allows multi-tree projects
to be specified, without being able to actually handle
trees within that multi-tree project; so for example,
you can only specify coreboot, and then it would run
on every coreboot tree.

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

include/vendor.sh

@@ -48,9 +48,7 @@ readkconfig()
 bootstrap()
 {
 	x_ ./mk -f coreboot ${cbdir##*/}
-	for d in uefitool biosutilities bios_extract; do
-		x_ ./mk -b "$d"
-	done
+	mk -b uefitool biosutilities bios_extract
 	[ -d "${kbc1126_ec_dump%/*}" ] && x_ make -C "$cbdir/util/kbc1126"
 	[ -n "$MRC_refcode_cbtree" ] && \
 	    cbfstoolref="elf/cbfstool/$MRC_refcode_cbtree/cbfstool" && \