6lowpan: add debugfs support

This patch will introduce a 6lowpan entry into the debugfs if enabled.
Inside this 6lowpan directory we create a subdirectories of all 6lowpan
interfaces to offer a per interface debugfs support.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
diff --git a/net/6lowpan/Makefile b/net/6lowpan/Makefile
index 9e35a5d..e44f3bf 100644
--- a/net/6lowpan/Makefile
+++ b/net/6lowpan/Makefile
@@ -1,6 +1,7 @@
 obj-$(CONFIG_6LOWPAN) += 6lowpan.o
 
 6lowpan-y := core.o iphc.o nhc.o
+6lowpan-$(CONFIG_6LOWPAN_DEBUGFS) += debugfs.o
 
 #rfc6282 nhcs
 obj-$(CONFIG_6LOWPAN_NHC_DEST) += nhc_dest.o