realtek: Enhance MTD/ECC kernel configuration for NAND targets

realtek: Enhance MTD/ECC kernel configuration for NAND targets

The Realtek NAND kernel configuration has some shortcomings.
Fix this as follows:

- MTD_NAND_ECC_REALTEK selects MTD_NAND_ECC and this selects
  MTD_NAND_CORE. For consistency add both config options.

- The partition layout of the Linksys switches requires some tricky
  concatenation to keep dual boot active. Add CONFIG_MTD_VIRT_CONCAT

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/20255
Signed-off-by: Robert Marko <robimarko@gmail.com>