Add support for plain URL in $(PKG)_PATCH variable

Until now, $(PKG)_PATCH allow only to download patches from same URL than tarball.
This patch allow to detect when plain URL are used in $(PKG)_PATCH and correctly
handle them.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>