AMD: Isolate AGESA and PI build environments

To backport features introduced with recent Chromebooks and/or Intel
boards in general, heavy work on the AMD AGESA platform infrastructure
is required. With the AGESA PI available in binary form only, community
members have little means to verify, debug and develop for the said
platforms.

Thus it makes sense to fork the existing agesawrapper interfaces, to give
AMD PI platforms a clean and independent sandbox. New directory layout
reflects the separation already taken place under 3rdparty/ and vendorcode/.

Change-Id: Ib60861266f8a70666617dde811663f2d5891a9e0
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7149
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
Tested-by: build bot (Jenkins)

src/include/cpu/amd/car.h

@@ -8,7 +8,7 @@ void post_cache_as_ram(void);
 void cache_as_ram_switch_stack(void *stacktop);
 void cache_as_ram_new_stack(void);
 
-#if CONFIG_CPU_AMD_AGESA
+#if CONFIG_CPU_AMD_AGESA || CONFIG_CPU_AMD_PI
 void disable_cache_as_ram(void);
 #endif