Allow to build romstage sources inside the bootblock

Having a separate romstage is only desirable:
 - with advanced setups like vboot or normal/fallback
 - boot medium is slow at startup (some ARM SOCs)
 - bootblock is limited in size (Intel APL 32K)

When this is not the case there is no need for the extra complexity
that romstage brings. Including the romstage sources inside the
bootblock substantially reduces the total code footprint. Often the
resulting code is 10-20k smaller.

This is controlled via a Kconfig option.

TESTED: works on qemu x86, arm and aarch64 with and without VBOOT.

Change-Id: Id68390edc1ba228b121cca89b80c64a92553e284
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55068
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/security/vboot/Kconfig

@@ -90,6 +90,7 @@ config VBOOT_STARTS_BEFORE_BOOTBLOCK
 config VBOOT_STARTS_IN_BOOTBLOCK
 	bool
 	default n
+	depends on SEPARATE_ROMSTAGE
 	help
 	  Firmware verification happens during the end of or right after the
 	  bootblock. This implies that a static VBOOT2_WORK() buffer must be