amdgpu driver wasn't loading because amdgpu firmware wasn't being
installed in the rootfs due to the wrong DEBIAN_ARCH variable.

rename ARCH to DEBIAN_ARCH also, so we don't have the confusing
DEBIAN_ARCH, KERNEL_ARCH and ARCH.
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Link: https://lore.kernel.org/r/20231024004525.169002-3-helen.koike@collabora.comSigned-off-by: Maxime Ripard <mripard@kernel.org>