trusted-firmware-a: provide symlinks for canonical names

Some platforms expect canonical names, like bl31.bin, instead of bl31-<plat>.bin
Provide symlinks for those as well.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>