ramips: rt288x: mark subtarget as source-only
Due to RAM/ROM size limitations, this sub-target has been unable
to generate usable images for a long time. Mark them as source-only
to save some resource for the buildbot.
Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18745
Signed-off-by: Robert Marko <[email protected]>