vboot2: Build verstage archive, then use that for building the stage

This slightly streamlines integrating the vboot2 library and
prepares for merging verstage and bootblock on selected devices.

Change-Id: I2163d1411d0c0c6bf80bce64796e1b6a5a02b802
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10004
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Makefile.inc

@@ -360,6 +360,10 @@ $(obj)/%.bootblock.o $(abspath $(obj))/%.bootblock.o: $(obj)/%.c $(obj)/config.h
 	@printf "    CC         $(subst $(obj)/,,$(@))\n"
 	$(CC_bootblock) -MMD $(CFLAGS_bootblock) $(CPPFLAGS_bootblock) $(bootblock-c-ccopts) -c -o $@ $<
 
+$(objgenerated)/libverstage.a: $$(verstage-objs)
+	rm -f $@
+	ar rcsT $@ $^
+
 #######################################################################
 # Clean up rules
 clean-abuild: