zephyr: boot: serial_recovery: Add no application entrace method

Adds a bootloader serial recovery entrance mode that will allow
recovering a module if there is no application that can be booted.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig.serial_recovery b/boot/zephyr/Kconfig.serial_recovery
index 5285e98..ddde2c6 100644
--- a/boot/zephyr/Kconfig.serial_recovery
+++ b/boot/zephyr/Kconfig.serial_recovery
@@ -185,6 +185,12 @@
 	  retention system (i.e. an application must set the boot mode to stay
 	  in serial recovery mode and reboot the module).
 
+config BOOT_SERIAL_NO_APPLICATION
+	bool "Stay in bootloader if no application"
+	help
+	  Allows for entering serial recovery mode if there is no bootable
+	  application that the bootloader can jump to.
+
 endmenu
 
 config BOOT_SERIAL_IMG_GRP_HASH