Support for multiple BR2_GLOBAL_PATCH_DIR

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>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>