Makefile: Switch to `.config` as input for the Ada `CB.Config`

So, this is odd in multiple ways. First of all, we fix something:
We work around a weirdness in `make oldconfig` that adds spurious
entries into the `auto.conf` for choices that were given a symbol
name.

When introducing the Ada config package, it seemed reasonable to
use `auto.conf` as source, but it turned out that we didn't use it
as input, only `config.h` and the original `.config` were used. As
the syntax for `.config` is the same as for `auto.conf` we use the
former now as input for Ada, too. One question remains: If `.config`
already contains all required information, what is this `auto.conf`
and what does it want?

Alternatively, we could try to fix `oldconfig` or add a linter to
forbid named choices. I thought, our build test would reject the
latter already. But the `oldconfig` behaviour is too subtle.

We keep a dependency on the `oldconfig` step, to make sure it runs
first.

Change-Id: If3fe6bc782251cdbd696395d3069a1c0bb0ae802
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36320
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Makefile

@@ -194,7 +194,7 @@ $(KCONFIG_AUTOHEADER): $(KCONFIG_CONFIG) $(objutil)/kconfig/conf
 $(KCONFIG_AUTOCONFIG): $(KCONFIG_AUTOHEADER)
 	true
 
-$(KCONFIG_AUTOADS): $(KCONFIG_AUTOCONFIG) $(objutil)/kconfig/toada
+$(KCONFIG_AUTOADS): $(KCONFIG_CONFIG) $(KCONFIG_AUTOHEADER) $(objutil)/kconfig/toada
 	$(objutil)/kconfig/toada CB.Config <$< >$@
 
 $(obj)/%/$(notdir $(KCONFIG_AUTOADS)): $(KCONFIG_AUTOADS)