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>

docs/manual/customize-packages.txt

@@ -0,0 +1,23 @@
+// -*- mode:doc -*- ;
+
+[[packages-custom]]
+Customizing packages
+~~~~~~~~~~~~~~~~~~~~
+
+It is sometimes useful to apply 'extra' patches to packages - over and
+above those provided in Buildroot. This might be used to support custom
+features in a project, for example, or when working on a new architecture.
+
+The +BR2_GLOBAL_PATCH_DIR+ configuration file option can be
+used to specify a directory containing global package patches.
+
+For a specific version <packageversion> of a specific package <packagename>,
+patches are applied as follows.
+
+First, the default Buildroot patch set for the package is applied.
+
+If the directory +$(BR2_GLOBAL_PATCH_DIR)/<packagename>/<packageversion>+
+exists, then all +*.patch+ files in the directory will be applied.
+
+Otherwise, if the directory +$(BR2_GLOBAL_PATCH_DIR)/<packagename>+
+exists, then all +*.patch+ files in the directory will be applied.

docs/manual/customize.txt

@@ -15,3 +15,5 @@ include::customize-kernel-config.txt[]
 include::customize-toolchain.txt[]
 
 include::customize-store.txt[]
+
+include::customize-packages.txt[]

docs/manual/patch-policy.txt

@@ -11,8 +11,9 @@ necessary to patch the source of the software to get it cross-built within
 Buildroot.
 
 Buildroot offers an infrastructure to automatically handle this during
-the builds. It supports two ways of applying patch sets: downloaded patches
-and patches supplied within buildroot.
+the builds. It supports three ways of applying patch sets: downloaded patches,
+patches supplied within buildroot and patches located in a user-defined
+global patch directory.
 
 Providing patches
 ~~~~~~~~~~~~~~~~~
@@ -45,6 +46,13 @@ application order.
 reference in their filename.
 - The field +<number>+ in the patch file name refers to the 'apply order'.
 
+Global patch directory
+^^^^^^^^^^^^^^^^^^^^^^
+
+The +BR2_GLOBAL_PATCH_DIR+ configuration file option can be
+used to specify a directory containing global package patches. See
+xref:packages-custom[] for details.
+
 
 How patches are applied
 ~~~~~~~~~~~~~~~~~~~~~~~
@@ -57,7 +65,7 @@ How patches are applied
   tarballs are applied;
 
 . If there are some +*.patch+ files in the package directory or in the
-  a package subdirectory named +<packagename>-<packageversion>+, then:
+  a package subdirectory named +<packageversion>+, then:
 +
 * If a +series+ file exists in the package directory, then patches are
   applied according to the +series+ file;