kbuild: document howto build external modules using several directories

Update modules.txt with info how to build external modules
with files in several directories.
The question popped up on lkml often enough to warrant this,
let's see if people read this stuff - or google hits it.

Signed-off-by: Sam Ravnborg <[e-mail]>

Documentation/kbuild/modules.txt

···
 	=== 5. Include files
 	   --- 5.1 How to include files from the kernel include dir
 	   --- 5.2 External modules using an include/ dir
+	   --- 5.3 External modules using several directories
 	=== 6. Module installation
 	   --- 6.1 INSTALL_MOD_PATH
 	   --- 6.2 INSTALL_MOD_DIR
···
 	Note that in the assignment there is no space between -I and the path.
 	This is a kbuild limitation:  there must be no space present.
 
+--- 5.3 External modules using several directories
+
+	If an external module does not follow the usual kernel style but
+	decide to spread files over several directories then kbuild can
+	support this too.
+
+	Consider the following example:
+	
+	|
+	+- src/complex_main.c
+	|   +- hal/hardwareif.c
+	|   +- hal/include/hardwareif.h
+	+- include/complex.h
+	
+	To build a single module named complex.ko we then need the following
+	kbuild file:
+
+	Kbuild:
+		obj-m := complex.o
+		complex-y := src/complex_main.o
+		complex-y += src/hal/hardwareif.o
+
+		EXTRA_CFLAGS := -I$(src)/include
+		EXTRA_CFLAGS += -I$(src)src/hal/include
+
+
+	kbuild knows how to handle .o files located in another directory -
+	although this is NOT reccommended practice. The syntax is to specify
+	the directory relative to the directory where the Kbuild file is
+	located.
+
+	To find the .h files we have to explicitly tell kbuild where to look
+	for the .h files. When kbuild executes current directory is always
+	the root of the kernel tree (argument to -C) and therefore we have to
+	tell kbuild how to find the .h files using absolute paths.
+	$(src) will specify the absolute path to the directory where the
+	Kbuild file are located when being build as an external module.
+	Therefore -I$(src)/ is used to point out the directory of the Kbuild
+	file and any additional path are just appended.
 
 === 6. Module installation