Support for multiple BR2_GLOBAL_PATCH_DIRS

Adding support for specifying multiple directories in
BR2_GLOBAL_PATCH_DIR. This will allow for a layered approach for the
patching of a package.

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
manual: update for multiple global patch dirs

Updating the documentation to reflect that multiple directories can
now be specified for BR2_GLOBAL_PATCH_DIR. Along with giving an
example use case of how to use multiple global patch directories.

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>