x86: Introduce RESET_VECTOR_IN_RAM option

Create a new Kconfig symbol that allows an x86 device to begin execution
when its reset vector is in DRAM and not at the traditional 0xfffffff0.

The implementation will follow later, this is just to setup various
ENV_xxx definitions correctly for the build environment.

Change-Id: I098ecf8bf200550db1e15f178f7661c1ac516dc5
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35004
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/symbols.h

@@ -72,7 +72,7 @@ DECLARE_REGION(bl31)
  * (Does not necessarily mean that the memory is accessible.) */
 static inline int preram_symbols_available(void)
 {
-	return !CONFIG(ARCH_X86) || ENV_CACHE_AS_RAM;
+	return !CONFIG(ARCH_X86) || ENV_ROMSTAGE_OR_BEFORE;
 }
 
 #endif /* __SYMBOLS_H */