rework patch model

rework patch model

At the Buildroot Developers Meeting (4-5 February 2013, in Brussels) a change
to the patch logic was discussed. See

http://elinux.org/Buildroot:DeveloperDaysFOSDEM2013

for details. In summary:

* For patches stored in the package directory, if
  package/<pkg>/<version>/ does exist, apply package/<pkg>/<version>/*.patch,
  otherwise, apply package/<pkg>/*.patch
* For patches stored in the global patches directory, if
  $(GLOBAL_PATCH_DIR)/<pkg>/<version>/ does exist, apply
  $(GLOBAL_PATCH_DIR)/<pkg>/<version>/*.patch, otherwise, apply
  $(GLOBAL_PATCH_DIR)/<pkg>/*.patch

This patch adds the new BR2_GLOBAL_PATCH_DIR configuration item, and reworks
the generic package infrastructure to implement the new patch logic.

[Peter: fixup doc nits as pointed out by Thomas]
Signed-off-by: Simon Dawson <spdawson@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>