kbuild: Add new Kbuild variable KBUILD_EXTRA_SYMBOLS

This patch adds a new (Kbuild) Makefile variable KBUILD_EXTRA_SYMBOLS.
The space separated list of file names assigned to KBUILD_EXTRA_SYMBOLS
is used when calling scripts/mod/modpost during stage 2 of the Kbuild
process for non-kernel-tree modules.

Signed-off-by: Richard Hacker <lerichi@gmx.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
index 1d247d5..1821c07 100644
--- a/Documentation/kbuild/modules.txt
+++ b/Documentation/kbuild/modules.txt
@@ -486,7 +486,7 @@
 	Sometimes, an external module uses exported symbols from another
 	external module. Kbuild needs to have full knowledge on all symbols
 	to avoid spitting out warnings about undefined symbols.
-	Two solutions exist to let kbuild know all symbols of more than
+	Three solutions exist to let kbuild know all symbols of more than
 	one external module.
 	The method with a top-level kbuild file is recommended but may be
 	impractical in certain situations.
@@ -523,6 +523,13 @@
 		containing the sum of all symbols defined and not part of the
 		kernel.
 
+	Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile
+		If it is impractical to copy Module.symvers from another
+		module, you can assign a space separated list of files to
+		KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be
+		loaded by modpost during the initialisation of its symbol
+		tables.
+
 === 8. Tips & Tricks
 
 --- 8.1 Testing for CONFIG_FOO_BAR