realtek: add xgs1210-12 b1 and switch to rt-loader

realtek: add xgs1210-12 b1 and switch to rt-loader

rev B1 is identical to rev A1 except for different PHYs on the 2.5gbps ports (lan9 and lan10)
Both revisions of xgs1210-12 are also switched to use rt-loader to avoid
problems due to overwriting the compressed image in memory when flashing
with the oem firmware (and also to save flash space with respect to gzip
compression)

Signed-off-by: Josh Bendavid <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20161
Signed-off-by: Hauke Mehrtens <[email protected]>