gstreamer1.0-libav: update git recipe to 1.7.1

gstreamer1.0-libav: update git recipe to 1.7.1

Move the ARM_INSTRUCTION_SET over-rides for armv4/armv5 out of the
1.6.3 recipe and into the common include, so that they also apply to
the git recipe.

The autogen.sh script takes care of cd'ing to the correct directory
before it does anything, so we don't need to cd on its behalf any more
in do_configure_prepend().

  http://cgit.freedesktop.org/gstreamer/gst-libav/tree/autogen.sh?id=1.7.1

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>