baremetal-image: Avoid overriding qemu variables from IMAGE_CLASSES

Since qemuboot is part of IMAGE_CLASSES via qemu.inc it is being
inherited before we set the QB_FOO variables.
Since our variables have conditional definitions and at that point
they've already been defined by qemuboot, we can no longer define
them in our class.

Move the IMAGE_CLASSES inherit to execute it after we set the
QB_FOO variables to fix booting via runqemu.

Signed-off-by: Alejandro Enedino Hernandez Samaniego <alejandro@enedino.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>