build.mk: Create an intermediate bare.img containing only a formated /usr.

This will allow the users to populate /usr with simples binaries like haproxy
and their config at runtime.
It's intented to be combined with the loader ability to run multiple binaries
in a sequential way.
For example -e '/tools/cpiod.so; /usr/haproxy.so -f /usr/haproxy.conf'

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>