Allow adding per-package override rsync exclusions

This allows using <PKG>_SRCDIR_OVERRIDE_RSYNC_EXCLUSIONS in local.mk to
skip copying parts of source trees unneeded for building. For example,
when developing WebKitGTK+, it's handy to skip copying all the tests and
other build directories, which are huge:

    WEBKITGTK_OVERRIDE_SRCDIR = /home/aperez/WebKit
    WEBKITGTK_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS = \
        --exclude JSTests --exclude ManualTests \
	--exclude PerformanceTests --exclude WebDriverTests \
	--exclude WebKitBuild --exclude WebKitLibraries \
	--exclude WebKit.xcworkspace --exclude Websites \
	--exclude Examples

This saves a good chunk of time when rsync is used for the first time to
copy the source tree over before building.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
[Arnout: move documentation to the end of the section]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

docs/manual/using-buildroot-development.txt

@@ -81,3 +81,19 @@ make busybox-rebuild all
 
 the root filesystem image in +output/images+ contains the updated
 BusyBox.
+
+Source trees for big projects often contain hundreds or thousands of
+files which are not needed for building, but will slow down the process
+of copying the sources with _rsync_. Optionally, it is possible define
++<pkg>_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS+ to skip syncing certain files
+from the source tree. For example, when working on the +webkitgtk+
+package, the following will exclude the tests and in-tree builds from
+a local WebKit source tree:
+
+------------------
+WEBKITGTK_OVERRIDE_SRCDIR = /home/bob/WebKit
+WEBKITGTK_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS = \
+	--exclude JSTests --exclude ManualTests --exclude PerformanceTests \
+	--exclude WebDriverTests --exclude WebKitBuild --exclude WebKitLibraries \
+	--exclude WebKit.xcworkspace --exclude Websites --exclude Examples
+------------------