The buildscript knows two different types of patches, which are
applied to pulled-in repositories:

1. Feed patches
Those are applied as "GIT patches" to the relevant repos,
directly after those have been checked out.
They reside in subfolders of the build_patches folder, and
have to be selected individually and manually in the
buildscript.sh.

2. Build patches
Those are applied later in the process, just using the system
patch tool, and changing the $target directory.
All patches in the folder "build_patches/openwrt" are read
and applied automatically.

This is both inconsistent (two different types of patches in
the same dir) and annoying (feed patches have to be specified
by hand), especially for unexperienced developers.

This patch addresses this by:
- separating files into two dirs: build_patches and feed_patches
- automatically scanning feed patches and thus having similar
  experience for the user (I cannot think of a case where we
  provide a patch, but do not use it)

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>