raspberrypi-usbboot: remove /usr from DESTDIR-based install commands

We have a patch that adds $(DESTDIR) to the install commands of
raspberrypi-usbboot, but it would still be installed in $(DESTDIR)/usr.
We don't want that, so remove the /usr part in the installation
commands.

Note that upstream has removed the 'install' target entirely, so
there's no point trying to keep the patch in upstreamable shape (i.e.
defaulting DESTDIR to /usr).

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