timestamps: Improve collection for ENV_ROMSTAGE_OR_BEFORE

Keep track of the active timestamp table location using
a CAR_GLOBAL variable. Done this way, the entire table
can be located outside _car_relocatable_data and we only
switch the pointer to CBMEM and copy the data before
CAR gets torn down.

Fix comments about requirements of timestamp_init() usage.

Remove timestamp_cache from postcar and ramstage, as CBMEM
is available early on.

Change-Id: I87370f62db23318069b6fd56ba0d1171d619cb8a
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35032
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/hardwaremain.c

@@ -461,9 +461,6 @@ void main(void)
 	 */
 	cbmem_initialize();
 
-	/* Record current time, try to locate timestamps in CBMEM. */
-	timestamp_init(timestamp_get());
-
 	timestamp_add_now(TS_START_RAMSTAGE);
 	post_code(POST_ENTRY_RAMSTAGE);