configs: meson64_android: implement AVB support

configs: meson64_android: implement AVB support

AVB (Android Verified Boot) is well supported in U-Boot already.
Add support for it in meson64_android.

This is controlled by the "force_avb" environment variable and the
CONFIG_CMD_AVB option.

Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>