zephyr: Enable interrupts unconditionally

Due to an issue described here:
https://github.com/zephyrproject-rtos/zephyr/issues/8393
interrupts are not enabled when multithreading is disabled.
Enable interrupts to allow the serial recovery mode UART to receive
characters.

Note: This commit must be reverted once the issue is addressed.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

boot/zephyr/serial_adapter.c

@@ -197,5 +197,10 @@ boot_uart_fifo_init(void)
 
 	uart_irq_rx_enable(uart_dev);
 
+	/* Enable all interrupts unconditionally. Note that this is due
+	 * to Zephyr issue #8393. This should be removed once the
+	 * issue is fixed in upstream Zephyr. */
+	irq_unlock(0);
+
 	return 0;
 }