compat: add COMPAT_BASE tag

compat: add COMPAT_BASE tag

I'm going to start adding tags to compat.git. This tag will
also be printed upon loading of the compat module. This is
useful to annotating what release of compat.git was used
exactly.

Moving forward the goal will be to try to add a tag only
once a full version of linux-next.git with a respective
date tag from that release has been addressed for backporting.

Right now the only consumer of compat.git that I am aware of
is compat-wireless (soon to be renamed to compat-drivers) so
what this means in practice is I will only add a tag to
compat.git once ckmake is used to verify compilation against
all supported kernels. This means there are two tasks when
backporting:

  * compat.git work
  * compat-wireless.git work

The compat.git work will likely be finished first than the
compat-wireless.git work. Older compat.git versions should
work with newer versions of tags from compat-wireless.git,
however, I may still add tags to compat.git to annotate
that that release (even if no changes were made) has also
been used and tested.

Every single release of compat.git and compat-wireless.git
that has a tag means that that tag has been tested with
ckmake to cross compile against all supported kernels.

Signed-off-by: Luis R. Rodriguez <mcgrof@frijolero.org>