arch/xtensa: custom configuration requires an overlay

Using a custom Xtensa configuration requires an overlay that provides
that configuration; not providing an overlay is like using the default
configuration, BR2_xtensa_fsf, so there would be no point in that case
in requesting a custom configuraiton.

Make providing an overlay mandatory for custom configurations.

Fixes:
    http://autobuild.buildroot.org/results/f0b/f0ba47d2534aeb3cc2921124aa639ae3aa072b9b

    xtensa-buildroot-linux-uclibc/bin/ld: ldso/ldso/ld-uClibc_so.a(ldso.oS): compiled for a big endian system and target is little endian

[1] https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=include/xtensa-config.h;hb=2ee5e4300186a92ad73f1a1a64cb918dc76c8d67#l28



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
[yann.morin.1998@free.fr:
  - always require an overlay for custom configurations, not just for
    little endian ones
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>