arch|cpu/x86: Add Kconfig option for x86 reset vector

Prepare for an implementation supporting the reset vector in RAM and
not the traditional 0xfffffff0.  Add a Kconfig symbol that can be used
in place of hardcoded values.

Change-Id: I6a814f7179ee4251aeeccb2555221616e944e03d
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37485
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/intel/fit/fit.ld

@@ -12,7 +12,7 @@
  */
 
 SECTIONS {
-	. = 0xffffffc0;
+	. = CONFIG_X86_RESET_VECTOR - 0x30; /* 0xffffffc0 */
 	.fit_pointer (.): {
 		KEEP(*(.fit_pointer))
 	}