soc/amd/picasso: Refactor transfer buffer check

The transfer buffer check had gotten large enough to deserve a function
of its own, so break it out.

BUG=None
TEST=Build
Branch=Zork

Signed-off-by: Martin Roth <martinroth@chromium.org>
Change-Id: Idf46f8edb6b70c63f623522e2bcd2f22d6d4790b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46112
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/picasso/Makefile.inc

@@ -24,6 +24,7 @@ bootblock-y += smi_util.c
 bootblock-y += config.c
 bootblock-y += pmutil.c
 bootblock-y += reset.c
+bootblock-$(CONFIG_VBOOT_STARTS_BEFORE_BOOTBLOCK) += bootblock/vboot_bootblock.c
 
 romstage-y += i2c.c
 romstage-y += romstage.c