scripts/config: fix *c_shipped build depency tracking

scripts/config: fix *c_shipped build depency tracking

The Makefile was missing dependencies on *c_shipped, so changes never
triggered a rebuild. Add these as optional dependencies so their absence
isn't treated as an error.

In addition, fix a typo preventing the zconf.lex.o from being removed on
clean.

Fixes: 9d5510a500a1 ("build: add new menuconfig code based on linux 3.9")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>