zephyr: Make makefile clearer about configuration
Separate the signature selection blocks, so that it is clearer to add
another config option for the boot verification.
# Makefile for building mcuboot as a Zephyr project.
-# These are the main configuration choices, mainly having to do with
-# what signature algorithm is desired. Choose one of the blocks
-# below, and uncomment the settings after it.
+# Configuration choices.
+
+#####################
+# Signature algorithm
+#####################
+# Choose one of RSA or ECDSA P-256 blocks, and uncomment the config
+# lines there, and comment out any other blocks.
-#####
# RSA
-#####
CONF_FILE = boot/zephyr/prj.conf
CFLAGS += -DBOOTUTIL_SIGN_RSA
-#############
# ECDSA P-256
-#############
#CONF_FILE = boot/zephyr/prj-p256.conf
#CFLAGS += -DBOOTUTIL_SIGN_EC256
+# Enable this option to have the bootloader verify the signature of
+# the primary image upon every boot. Without it, signature
+# verification only happens on upgrade.
+CFLAGS += -DBOOTUTIL_VALIDATE_SLOT0
+
##############################
# End of configuration blocks.
##############################
