Add public/private key file option to Kconfig

Also adds instrutions and defaults to prj.conf.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 10eb22d..96b5268 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -38,6 +38,14 @@
 
 endchoice
 
+config BOOT_SIGNATURE_KEY_FILE
+	string "PEM key file"
+	default ""
+	help
+	  The key file will be parsed by imgtool's getpub command and a .c source
+	  with the public key information will be written in a format expected by
+	  MCUboot.
+
 config MBEDTLS_CFG_FILE
 	default "mcuboot-mbedtls-cfg.h"