genromfs: use $(HOST_DIR) instead of $(HOST_DIR)/usr as prefix

genromfs is special because it uses "PREFIX" in the meaning of DESTDIR
and "prefix" in the meaning of prefix. We were up to know using it
incorrectly for host: PREFIX shouldn't be set and only prefix should
be set.

Add an explanatory comment for this unusual behaviour.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>