arch/Config.in.xtensa: provide BR2_ENDIAN symbol

Packages get hints from the buildroot at configure time, e.g.
ac_cv_c_bigendian=yes/no. This particular hint is based on the value of
BR2_ENDIAN and when it doesn't match actual toolchain endiannes the
build may break.
Provide BR2_ENDIAN for xtensa to fix this sort of build errors.

Fixes:
  http://autobuild.buildroot.net/results/31115fe8d88f52d77ed0f2da769eb8896a1b34a2/



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