bitbake.conf, image.bbclass: Add and use USERDISTRO var to preserve original DISTRO...

bitbake.conf, image.bbclass: Add and use USERDISTRO var to preserve original DISTRO value.
* Some distros override DISTRO var to hack overrides right, etc. Yet, original
value may be needed sometimes. For example, bbimage expects DISTRO to be what
user set/passed it, not something else.
* TODO: After interim cleanup during preparation of this patch, it turns out
that angstrom is the only distro to (ab)use DISTRO overriding. It may make sense
to instead get its OVERRIDES value right.