util/kconfig: add olddefconfig target

There doesn't seem to have been an olddefconfig target in the coreboot
version of the Kconfig makefile.  It's listed in the .PHONY, but it
doesn't seem like it's ever been there.  This is useful for expanding
a miniconfig saved with 'make savedefconfig'.

Change-Id: I3798f8469135b58d32da68d4b0e434ab5351b501
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: http://review.coreboot.org/11273
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/kconfig/Makefile

@@ -97,6 +97,9 @@ else
 	$< --defconfig=$(KBUILD_DEFCONFIG) $(Kconfig)
 endif
 
+olddefconfig: $(objk)/conf
+	$< --olddefconfig $(Kconfig)
+
 %_defconfig: $(objk)/conf
 	$< --defconfig=configs/$@ $(Kconfig)