soc/amd/piasso,cezanne: add warning about using all-y in Makefile.inc

all-y will also add a compilation unit to the verstage on PSP build that
runs on an ARM code instead of a x86 one. At the moment Cezanne doesn't
have verstage on PSP support yet, but since it'll eventually land it
doesn't hurt to already add the comment now.

Change-Id: I15fb66e796cab48737ba5ac463c4c973794a005a
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48521
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/soc/amd/cezanne/Makefile.inc

@@ -2,6 +2,7 @@
 
 ifeq ($(CONFIG_SOC_AMD_CEZANNE),y)
 
+# Beware that all-y also adds the compilation unit to verstage on PSP
 all-y += config.c
 
 bootblock-y	+= bootblock.c

src/soc/amd/picasso/Makefile.inc

@@ -9,6 +9,7 @@ subdirs-y += ../../../cpu/x86/pae
 subdirs-y += ../../../cpu/x86/smm
 subdirs-$(CONFIG_VBOOT_STARTS_BEFORE_BOOTBLOCK) += psp_verstage
 
+# Beware that all-y also adds the compilation unit to verstage on PSP
 all-y += config.c
 all-y += aoac.c