linux: apply xtensa overlay

Xtensa core configuration must be added to linux before it can be
built for that xtensa CPU variant. Extract configuration files from the
xtensa overlay as is done for other packages that need to be configured
for a specific xtensa core.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>