kbuild: make it possible to specify the module output dir

Make modinst_dir user-defined on the command line.

This allows to do things like:
make MODLIB=output/ modinst_dir=. modules_install

to ensure all the .ko are in the output/ directory.

Change-Id: I2bc007eea27ee744d35289e26e4a8ac43ba04151
Signed-off-by: Rom Lemarchand <romlem@android.com>
diff --git a/scripts/Makefile.modinst b/scripts/Makefile.modinst
index 07650ee..6f4c3f5 100644
--- a/scripts/Makefile.modinst
+++ b/scripts/Makefile.modinst
@@ -29,7 +29,7 @@
 INSTALL_MOD_DIR ?= extra
 ext-mod-dir = $(INSTALL_MOD_DIR)$(subst $(patsubst %/,%,$(KBUILD_EXTMOD)),,$(@D))
 
-modinst_dir = $(if $(KBUILD_EXTMOD),$(ext-mod-dir),kernel/$(@D))
+modinst_dir ?= $(if $(KBUILD_EXTMOD),$(ext-mod-dir),kernel/$(@D))
 
 $(modules):
 	$(call cmd,modules_install,$(MODLIB)/$(modinst_dir))