zephyr: selectable serial device for recovery

This commit adds a choice for the device to use for serial recovery.

Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

fixup
diff --git a/boot/zephyr/serial_adapter.c b/boot/zephyr/serial_adapter.c
index 92aa6c6..fc39f71 100644
--- a/boot/zephyr/serial_adapter.c
+++ b/boot/zephyr/serial_adapter.c
@@ -188,7 +188,11 @@
 static int
 boot_uart_fifo_init(void)
 {
+#ifdef CONFIG_BOOT_SERIAL_UART
 	uart_dev = device_get_binding(CONFIG_UART_CONSOLE_ON_DEV_NAME);
+#elif CONFIG_BOOT_SERIAL_CDC_ACM
+	uart_dev = device_get_binding(CONFIG_CDC_ACM_PORT_NAME);
+#endif
 	u8_t c;
 
 	if (!uart_dev) {