util/nvramtool: Create nvramtool object directories earlier

The existing rule created a potential race condition between creating
the directory and putting files in there, so use our existing
infrastructure for directory creation instead.

Change-Id: If52a9f558c7d9ce85f71ba53232594699c9d357a
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37798
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Jett Rink <jettrink@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/nvramtool/Makefile.inc

@@ -36,8 +36,7 @@ NVRAMTOOLLDFLAGS += -lregex -lwsock32
 nvramtoolobj += win32mmap.o
 endif
 
-$(objutil)/nvramtool $(objutil)/nvramtool/accessors $(objutil)/nvramtool/cli:
-	mkdir -p $@
+additional-dirs += $(objutil)/nvramtool/accessors $(objutil)/nvramtool/cli
 
 $(objutil)/nvramtool/%.o: $(top)/util/nvramtool/%.c
 	printf "    HOSTCC     $(subst $(objutil)/,,$(@))\n"