board: ti: keys: add .key and .crt for fit signature signing

Fit signature mechanism through the standards require the presence of
.key and .crt in the folder with the same name, since we are using our
custMpk only for the signing, update the format to that of standards to
be compatible for packaging easily.

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>