env: ti: mmc.env: Fix overlays directory path

Similar to get_fdt_mmc make get_overlays_mmc look at /boot/dtb/* path
for overlay files.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Reviewed-by: Nikhil M Jain <n-jain1@ti.com>