boot: zephyr: Add pin reset serial recovery entrance method

Adds an optional method for entering serial recovery mode by use
of a pin reset.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig.serial_recovery b/boot/zephyr/Kconfig.serial_recovery
index ddde2c6..c73573b 100644
--- a/boot/zephyr/Kconfig.serial_recovery
+++ b/boot/zephyr/Kconfig.serial_recovery
@@ -191,6 +191,13 @@
 	  Allows for entering serial recovery mode if there is no bootable
 	  application that the bootloader can jump to.
 
+config BOOT_SERIAL_PIN_RESET
+	bool "Check for device reset by pin"
+	select HWINFO
+	help
+	  Checks if the module reset was caused by the reset pin and will
+	  remain in bootloader serial recovery mode if it was.
+
 endmenu
 
 config BOOT_SERIAL_IMG_GRP_HASH