rampis: mt7621: Re-enable Edgerouter-X image builds

rampis: mt7621: Re-enable Edgerouter-X image builds

With the new layout providing 6MB for kernel there will be no issues
with kernel size affecting build of images.

Re-enable image builds for Edgerouter-X and X-SFP.

Signed-off-by: Tim Lunn <tim@feathertop.org>
Tested-by: Mauri Sandberg <maukka@ext.kapsi.fi>
Link: https://github.com/openwrt/openwrt/pull/15194
(cherry picked from commit 71b4842e3bdaed91cd2daa4b28a208559c3de7c6)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>