arch/x86/Kconfig: Hide the prefix option on all but BOOTBLOCK_NORMAL

Change-Id: Icf5e8fa18bea1cdfb85b8a4999d8fccea94d16b9
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33324
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/arch/x86/Kconfig

@@ -247,6 +247,7 @@ config BOOTBLOCK_SIMPLE
 	bool "Always load fallback"
 
 config BOOTBLOCK_NORMAL
+	select CONFIGURABLE_CBFS_PREFIX
 	bool "Switch to normal if CMOS says so"
 
 endchoice