sanity.bbclass: fix splitting on newlines of SANITY_TESTED_DISTROS

sanity.bbclass: fix splitting on newlines of SANITY_TESTED_DISTROS

This variable should be split with \n sequences and these need to be
specified literally in the string. A corrected version of the example
given in the original commit (OE-core rev
75e3875341ddc8940e9ee2ccbbb2ec18194a68e6):

SANITY_TESTED_DISTROS = " \
        Ubuntu 11.04 \n \
        Fedora release 14 (Laughlin) \n \
        "

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>