configs: meson: remove unnecessary MESON_FDTFILE_SETTING

MESON_FDTFILE_SETTING is used to define the default name of the fdt
file in u-boot environment. We can easily derive it from
CONFIG_DEFAULT_DEVICE_TREE instead. This will help factorize the code
a bit

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>