linux: Add Linux Kernel extensions menu

Add a kernel sub-menu (called "Linux Kernel Extensions"), which makes
possible to patch it or tweak the kernel build step.

 * All linux/linux-ext-*.mk files will be read by the make process.
 * The menu can be customized in "linux/Config.ext.in".

[Peter: small fixes]
Signed-off-by: Benoit Mauduit <benoit.mauduit@openwide.fr>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

linux/Config.ext.in

@@ -0,0 +1,3 @@
+menu "Linux Kernel Extensions"
+
+endmenu

linux/Config.in

@@ -176,6 +176,10 @@ config BR2_LINUX_KERNEL_INSTALL_TARGET
 	  /boot in the target root filesystem, as is typically done on
 	  x86/x86_64 systems.
 
+
+# Linux extensions
+source "linux/Config.ext.in"
+
 endif # BR2_LINUX_KERNEL
 
 endmenu

linux/linux.mk

@@ -177,6 +177,8 @@ define LINUX_INSTALL_TARGET_CMDS
 	fi
 endef
 
+include linux/linux-ext-*.mk
+
 $(eval $(call GENTARGETS,,linux))
 
 linux-menuconfig linux-xconfig linux-gconfig linux-nconfig linux26-menuconfig linux26-xconfig linux26-gconfig linux26-nconfig: dirs $(LINUX_DIR)/.stamp_configured