configs: add new configuration for qemu-system-xtensa

Tested with Qemu 2.0.0.
Overlay is from
https://github.com/jcmvbkbc/xtensa-toolchain-build/tree/master/overlays



Many thanks to Max Filippov <jcmvbkbc@gmail.com> for his extended help
and kernel config examples.

A small kernel patch is required, because mkimage does not contain
support for Xtensa and the uImage target is failing. Redboot is unused
so disable it, too.

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