build.mk: Rename usr.raw to bare.raw

We have an intermediate bare.img now. Rename usr.raw to bare.raw to make the
naming more consistent: bare.raw -> bare.img -> usr.img.

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