Intel: Add common header file for CAR setup

When passing '-ffreestanding' the 'main' romstage.c may no longer
necessarily be considered the entry point.

From the C specification in 5.1.2.1 Freestanding environment;

"In a freestanding environment (in which C program execution may take
place without any benefit of an operating system), the name and type of
the function called at program startup are implementation-defined."

Clang complains about these being missing as Clang is somewhat more
strict about the spec than GNU/GCC is. An advantage here is that a
different entry-point type-signature shall now be warned about at
compile time.

Change-Id: I467001adabd47958c30c9a15e3248e42ed1151f3
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5872
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>