ramips: Introduce CONFIG_MIPS_APPENDED_RAW_DTB to F5D8235 V1 (rt288x)

This activates the CONFIG_MIPS_APPENDED_RAW_DTB kernel configuration and
configures the F5D8235 V1 target to use append-dtb for kernel creation
instead of OWRTDTB.

Signed-off-by: Tobias Wolf <github-NTEO@vplace.de>
[Jo-Philipp Wich: rewrap commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>