Makefile: add modules_install target

Add a `modules_install` target to the Makefile to assist with
installation of the built modules. This target is a useful shortcut for
local development and is also used by build systems such as
OpenEmbedded.

Signed-off-by: Tom Stokes <tomstokes@radixengineering.com>

Author: tomstokes

Makefile

@@ -4,3 +4,6 @@ KDIR ?= /lib/modules/`uname -r`/build
 
 default:
 	$(MAKE) -C $(KDIR) M=$$PWD
+
+modules_install: default
+	$(MAKE) -C $(KDIR) M=$$PWD modules_install