backports: dissolve backport.h

backports: dissolve backport.h

We'll resolve all the other compat-*.h files next.

To keep it compiling, we now also need to remove all
the pr_fmt patches (they add printk.h which can't be
included as the first header file any more.)

I attempted to simply not include anything from the
command line, but very old kernels (e.g. 2.6.24)
don't include kconfig.h from the command line but
include/linux/autoconf.h directly, and thus won't
get our override. Thus we need to keep a very small
backport.h.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>