How to use DTB overlays?

[Vincent DEFERT <vincent.defert%posteo.net@localhost>]

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";
        };
    };
};