realtek: Add initial kernel config for RTL838x sub-target

Move the generic kernel configs to the rtl838x sub-target.

Signed-off-by: Birger Koblitz <git@birger-koblitz.de>

diff --git a/target/linux/realtek/config-5.10 b/target/linux/realtek/rtl838x/config-5.10
similarity index 100%
rename from target/linux/realtek/config-5.10
rename to target/linux/realtek/rtl838x/config-5.10