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/Kconfig

@@ -243,6 +243,14 @@ config COMPRESS_BOOTBLOCK
 	  user-selectable. (There's no real point in offering this to the user
 	  anyway... if it works and saves boot time, you would always want it.)
 
+config SEPARATE_ROMSTAGE
+	bool "Build a separate romstage"
+	default y
+	help
+	  Build a separate romstage that is loaded by bootblock. With this
+	  option disabled the romstage sources are linked inside the bootblock
+	  as a single stage.
+
 config INCLUDE_CONFIG_FILE
 	bool "Include the coreboot .config file into the ROM image"
 	# Default value set at the end of the file
@@ -1523,6 +1531,7 @@ config HAVE_VERSTAGE
 
 config HAVE_ROMSTAGE
 	bool
+	depends on SEPARATE_ROMSTAGE
 	default y
 
 config HAVE_RAMSTAGE