configs: microchip_mpfs_icicle: set DEFAULT_FDT_FILE

Variable $fdtfile needs to be set for automatically loading a device-tree
from the ESP or boot partition.

* Set CONFIG_DEFAULT_FDT_FILE in the defconfig.
* Add $fdtfile to the default environment.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
2 files changed