Turn on -Wcontext by default

Turn on -Wcontext by default

Context checking currently requires explicitly passing the -Wcontext option.
In a Linux build, this requires passing CF=-Wcontext.  Thus, running people
running sparse over Linux or other software in the default way will not see
context warnings.  Change this by enabling -Wcontext.

While context checking does generate false positives, due to both missing
annotations on kernel functions and insufficiently clever checking in sparse,
the context warnings still do provide useful information and help track down
real bugs.  Furthermore, showing these warnings by default may incite more
people to add annotations or enhance sparse.  As a last resort, -Wno-context
will still work.

Signed-off-by: Josh Triplett <josh@freedesktop.org>