linux-yocto: relocate KERNEL_FEATURES_RISCV

linux-yocto: relocate KERNEL_FEATURES_RISCV

In a custom linux recipe that requires linux-yocto.inc, the metadata
files (*.scc) end up in KERNEL_FEATURES. However, the custom recipe's
FILESPATH does not contain the path to oe-core's linux-yocto directory
(as expected), which leads to the error below:

ERROR: linux-custom-6.12.15+git-r0 do_kernel_metadata:
Feature 'arch/riscv/tunes/riscv-isa-clear.scc' not found, this will cause configuration failures.
ERROR: linux-custom-6.12.15+git-r0 do_kernel_metadata:
Check the SRC_URI for meta-data repositories or directories that may be missing

Move KERNEL_FEATURES_RISCV - along with the appends - to a new file:
linux-yocto-features.inc.

Signed-off-by: João Marcos Costa <joaomarcos.costa@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>