backports: add support for backport_srctree

backports: add support for backport_srctree

The Linux kernel uses srctree to help prefix header files
and scripts. When backporting we want to be able to override
driver's Makefiles' usage of srctree with our own. Since we
can't easily and sanely override the srctree itself without
causing a lot of issues just use the python generator to
do the change for us on the target Makefiles. This addresses
the srctree usage case and where ccflags was used without
prefixing any parent directory when used with drivers, the
kernel tends to fix that with addtree command.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
[also remove now unnecessary patches]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>