sanity.bbclass: add optional untested host distro warning

sanity.bbclass: add optional untested host distro warning

SANITY_TESTED_DISTROS, if specified, is expected to be a
newline-delimited list of distro identifier strings, e.g.

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

(spaces, tabs etc. are trimmed)

If SANITY_TESTED_DISTROS is defined, we will attempt to detect the host
distribution. If the distribution is not in SANITY_TESTED_DISTROS or we
could not detect the distribution then we show a warning during sanity
checking.

Provides the mechanism for fixing [YOCTO #1096].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>