Reorganize zephyr kconfig

This patch isolates mcuboot settings under `MCUBoot`
menu in menuconfig view. This improves legibility.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index bc3c234..87664ec 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -21,6 +21,8 @@
 	help
 	  Use TinyCrypt for crypto primitives.
 
+menu "MCUBoot settings"
+
 choice
 	prompt "Signature type"
 	default BOOT_SIGNATURE_TYPE_RSA
@@ -121,6 +123,8 @@
 
 endif # MCUBOOT_SERIAL
 
+endmenu
+
 config MCUBOOT_DEVICE_SETTINGS
 	# Hidden selector for device-specific settings
 	bool