mkamsboot: convert to use libtools.make.

As a result building mkamsboot for Rockbox Utility will now respect BUILD_DIR,
which should eliminate all possibly remaining issues with dualboot.o.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@31283 a1c6a512-1295-4272-9138-f99709370657
1 file changed