bitbake.conf: Default DISTRO to nodistro

bitbake.conf: Default DISTRO to nodistro

An empty distro value leads to OVERRIDES and FILESOVERRIDES containing
"::" entries which causes odd issues such as files being included when
they shouldn't be. We could put in anonymous python to guard against
empty entries but its messy and setting a default value for DISTRO to
something harmless is much easier.

This patch adds a weak default and ensures the sanity test doesn't
complain about it.

DISTRO_VERSION and SDK_VERSION are also updated to match.

(From OE-Core rev: b7279f99639774674da806d37d252f388f33055f)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>