soc/intel/braswell: Use common cpu/intel/car code

The code in cpu/intel/car/romstage.c Does most of the things like
setting up timestamps, stack guards, entering postcar.

A functional difference is that the FSP header is searched for twice
instead of passed from the CAR entry to the C code. When using
C_ENVIRONMENT_BOOTBLOCK this needs to be done anyway (or a special
linker symbol kept across multiple stages is needed, which is likely
not worth the speedup).

Change-Id: I0f03e5a808f00157fdd807b104417a54e4bde7b2
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32963
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/soc/intel/braswell/romstage/Makefile.inc

@@ -1,2 +1,3 @@
+romstage-y += ../../../../cpu/intel/car/romstage.c
 romstage-y += pmc.c
 romstage-y += romstage.c