zephyr: Add simple benchmark support to Zephyr

This adds a Kconfig feature CONFIG_BOOT_USE_BENCH, which when enabled
will print the simple benchmarking output as log messages.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index f93f7f9..1bf3eb8 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -206,6 +206,15 @@
 
 	  This is not available for all targets.
 
+config BOOT_USE_BENCH
+        bool "Enable benchmark code"
+        default n
+        help
+          If y, adds support for simple benchmarking that can record
+          time intervals between two calls.  The time printed depends
+          on the particular Zephyr target, and is generally ticks of a
+          specific board-specific timer.
+
 module = MCUBOOT
 module-str = MCUBoot bootloader
 source "subsys/logging/Kconfig.template.log_config"