configs: mediatek: enable boot via extlinux

Enable FAT and SYSBOOT to use extlinux boot script

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: Julien STEPHAN <jstephan@baylibre.com>