Makefile.inc: Use additional-dirs for $(objcbfs), $(objgenerated)

We use `additional-dirs` for a single `mkdir -p` invocation for all
directiories. I don't see why these two, $(objcbfs) and $(objgenerated),
should be an exception.

Fixes clean builds for targets that don't include the phony `coreboot`
target, e.g. `make qemu`.

Change-Id: I85abaa74cddefd2bd669e2b5c8934352775070fe
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51318
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Makefile

@@ -13,6 +13,8 @@ objutil ?= $(obj)/util
 objk := $(objutil)/kconfig
 absobj := $(abspath $(obj))
 
+additional-dirs :=
+
 VBOOT_HOST_BUILD ?= $(abspath $(objutil)/vboot_lib)
 
 COREBOOT_EXPORTS := COREBOOT_EXPORTS