src/soc: Get rid of most src/soc/Kconfig files

Most of the src/soc/Kconfig files are only there for AMD and Intel to
load the main SoC Kconfig files before any common files.  That can be
done in src/Kconfig instead.  Moving the loads to the lower level allows
the removal of all but the Intel soc/Kconfig file, which can be removed
in a follow-on patch.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I5061191fe23e0b7c745e90874bd7b390806bbcfa
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65327
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

src/soc/example/Kconfig

@@ -1 +0,0 @@
-source "src/soc/example/*/Kconfig"