boot_serial: Add optional img mgmt slot info feature

Adds a minimal version of the slot info feature to serial recovery,
and enables it by default.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig.serial_recovery b/boot/zephyr/Kconfig.serial_recovery
index fe82697..7ed9a7d 100644
--- a/boot/zephyr/Kconfig.serial_recovery
+++ b/boot/zephyr/Kconfig.serial_recovery
@@ -202,4 +202,11 @@
 	  If y, image states will be included with image lists and the set
 	  state command can be used to mark an image as test/confirmed.
 
+config BOOT_SERIAL_IMG_GRP_SLOT_INFO
+	bool "Slot info"
+	default y if UPDATEABLE_IMAGE_NUMBER > 1
+	help
+	  If y, will include the slot info command which lists what available
+	  slots there are in the system.
+
 endif # MCUBOOT_SERIAL