imx-uxb-loader: use prefix instead of DESTDIR for host installation

prefix defaults to /usr, so setting DESTDIR installs things in
$(HOST_DIR)/usr.

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