ELECOM WRC-X3200GST3 uses the same header/footer as WRC-GS/GST devices
in ramips/mt7621 subtarget, so move "Build/elecom-wrc-gs-factory" to
image-commands.mk to use from mediatek/mt7622 subtarget.
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>