configs: ti: Fix usage of undefined variable overlay_files

The env variable overlay_files is used while adding the environment
support commands for FIT loading, but it was cleaned up just prior in
commit ee53b595 ("configs: Remove unneeded overlay_files environment
variable"). Fix this by replacing the undefined variable with the
proper name_overlays env variable.

Fixes: 76470b69 ("configs: ti: Add environment support commands for FIT loading")
Signed-off-by: Suman Anna <s-anna@ti.com>