support/scripts/gen-bootlin-toolchains: correct xtensa-lx60 toolchain dependencies

support/scripts/gen-bootlin-toolchains: correct xtensa-lx60 toolchain dependencies

Fixes:
http://autobuild.buildroot.net/results/011/0111c2ed54618daaeedfc66b0ea04eda00a7e855/
http://autobuild.buildroot.net/results/e53/e53e3880b63a23fa3b3e6d34664d40d5ddbdff89/
..

As listed in the br_fragment file of the toolchain, this is built for a
little-endian "custom" xtensa variant rather than the (big-endian) fsf one:

BR2_xtensa=y
BR2_XTENSA_CUSTOM=y

So update the dependencies in the script and regenerate Config.in.options /
toolchain test.  Also fixup the autobuild config snippet to match.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>