nvidia/cbootimage: avoid upstream's build system

It brings in useless dependencies, a weird autotools
configuration, and tons of pain everywhere.

Instead just build things ourselves.

Change-Id: I67f06e711cb9dcd594363bc1a4f99d3273074549
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6986
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
diff --git a/Makefile.inc b/Makefile.inc
index 382eead..d307c60 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -308,14 +308,7 @@
 
 CBOOTIMAGE:=$(objutil)/cbootimage/cbootimage
 
-$(CBOOTIMAGE): $(top)/util/nvidia-cbootimage/Makefile
-	@printf "    MAKE     $(subst $(obj)/,,$(@))\n"
-	$(MAKE) -C $(top)/util/nvidia-cbootimage
-	cp $(top)/util/nvidia-cbootimage/src/cbootimage $@
-
-$(top)/util/nvidia-cbootimage/Makefile:
-	cd $(top)/util/nvidia-cbootimage/; \
-	./autogen.sh >/dev/null
+subdirs-y += util/nvidia
 
 #######################################################################
 # needed objects that every mainboard uses