boot: zephyr: Use private key

Change adds using own private key for MCUBoot bootloader.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 10cd885..f1ec0a7 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -128,6 +128,11 @@
 	string "PEM key file"
 	default ""
 	help
+	  You can use either absolute or relative path.
+	  In case relative path is used, the build system assumes that it starts
+	  from the directory where the MCUBoot KConfig configuration file is
+	  located. If the key file is not there, the build system uses relative
+	  path that starts from the MCUBoot repository root directory.
 	  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.