utils/genrandconfig: disallow configs with BR2_XTENSA_CUSTOM=y

When BR2_XTENSA_CUSTOM=y is used with the internal toolchain, an
overlay file is mandatory, which genrandconfig can't provide. So we
simply disallow such configurations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>