arch/x86: Fix payload loading via bounce-buffer

Fix regression (supposedly) after commit:

  23d62dd lib/bootmem: Add more bootmem tags

Without RELOCATABLE_RAMSTAGE, payload is allowed to overwrite
memory regions of the running ramstage. This case is handled
gracefully via a bounce-buffer implementation in arch/x86/boot.c.

Change-Id: I1c9bbdb963a7210d0817a7a990a70a1e4fc03624
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/26935
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/arch/x86/boot.c

@@ -203,7 +203,7 @@ static void jmp_payload(void *entry, unsigned long buffer, unsigned long size)
 
 int arch_supports_bounce_buffer(void)
 {
-	return 1;
+	return !IS_ENABLED(CONFIG_RELOCATABLE_RAMSTAGE);
 }
 
 int payload_arch_usable_ram_quirk(uint64_t start, uint64_t size)