How to use DTB overlays?

As meson-gxbb-odroidc2.dts doesn't enable all the peripherals available 
on the ODROID C2's GPIO connector, I want to create an overlay to enable 
the missing peripherals.
I have placed it in /boot/dtb/amlogic alongside meson-gxbb-odroidc2.dtb, 
but it is just ignored.
What am I doing wrong?
Here's my overlay source:

/*
  * Overlay enabling I2C B, UART A on the ODROID C2's GPIO connector.
  *
  * I2C B is configured to operate at 400 kHz while I2C A operates at
  * 100 kHz so the user can take advantage of faster peripherals.
  *
  * Build command:
  * nbdtc -O dtb -o meson-gxbb-odroidc2-ext.dtbo meson-gxbb-odroidc2-ext.dts
  */

/dts-v1/;
/plugin/;

/ {
     compatible = "hardkernel,odroid-c2";

     /*
      * Enable I2C B at 400 kHz.
      *
      * Uses GPIODV.BIT26 on GPIO connector pin 27 for SDA
      * and GPIODV.BIT27 on GPIO connector pin 28 for SCL.
      */
     fragment@0 {
         target = <&i2c_B>;
         __overlay__ {
             status = "okay";
             pinctrl-0 = <&i2c_b_pins>;
             pinctrl-names = "default";
             clock-frequency = <400000>;
         };
     };

     /*
      * Enable UART A.
      *
      * Uses GPIOX.BIT12 on GPIO connector pin 8 for TX
      * and GPIOX.BIT13 on GPIO connector pin 10 for RX.
      */
     fragment@1 {
         target = <&uart_A>;
         __overlay__ {
             status = "okay";
             pinctrl-0 = <&uart_a_pins>;
             pinctrl-names = "default";
         };
     };
};