build.mk: Fix bootfs and usr manifest dependencies

Commit 0dcf1f8 ("OSv module support") didn't add a dependency to
bootfs.manifest.skel and usr.manifest.skel which causes image not to be
rebuilt if the files are changed.

Fix that up.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc/pthread.cc: Switch to WARN_STUBBED()

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>