lantiq/xrx200: move 8M flash devices to a small flash subtarget

lantiq/xrx200: move 8M flash devices to a small flash subtarget

Images for xrx200 8M flash are either not building due to image
size (TD-W8970, TD-W8980) or building such that the available
free space in the overlayfs is too little to be useful.

To keep images for these devices buildable, move them into a
small flash variant of the xrx200 subtarget.  As these devices
are NOR flash only, remove NAND and UBI references from the
kernel config to gain some additional image size reduction.

The apparent 8M flash devices Arcadyan VGV7510KW22-brn,
Arcadyan VGV7519-brn and Lantiq Easy80920-nor seem to exist in
order to create special "factory" installation images for these
devices (which actually have larger flash: 16MB for the
Arcardyan devices; 64MB for the Lantiq device).  As a
considerable amount of surgery would appear to be required to
the uboot-lantiq package structure to separate the "factory"
from the "sysupgrade" device recipes for these devices they
remain in the xrx200 target - if factory images aren't now
created, 23.05.x factory images should suffice for initial
installation.

Tested on: Netgear DM200, TP-Link TD-W8980,
           AVM Fritz7490 (xrx200 subtarget: image build only)

Fixes: https://github.com/openwrt/openwrt/issues/16761
Signed-off-by: Andrew MacIntyre <andymac@pcug.org.au>
Link: https://github.com/openwrt/openwrt/pull/17113
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>