package/gst-omx: fix gstomx.conf installation for raspberry

package/gst-omx: fix gstomx.conf installation for raspberry

The commit 7e6f4e5 introduces a new bug when building the gst-omx package
for the raspberry.

GST_OMX_VARIANT variable in gst-omx.mk was shadowed to 'generic' after being
set well for raspberry. It results having the gstomx.conf not being installed,
and thus having gst-omx installed but none of its features available.

Signed-off-by: Augustin Thiercelin <augustin.thiercelin@outlook.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>