realtek: add NAND targets for RTL93xx

realtek: add NAND targets for RTL93xx

Some known RTL93xx devices like the Linksys LGS328C or LGS352C are
NAND based. These require additional drivers and packages (e.g. UBI).
The current subtargets are already taylored down for devices with
only 16MB flash. Adding features that are not used will only make
the storage situation more complicated.

Add two new subtargets for RTL93xx that include the basic NAND, UBI
and MTD features. To achieve this do the following:

- Create new subtarget folders
- Copy the existing config and makefiles over
- Add the basic additional features
- Mark them as SOURCE-ONLY
- Add empty image makefiles
- Remove unneded NAND/MTD features from existing configs

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19700
Signed-off-by: Robert Marko <robimarko@gmail.com>