pkg-generick.mk: support for multiple 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.
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.