boot/arm-trusted-firmware: ignore licencing check for user defined official version

The commit [1] "licensing info is only valid for v1.4" fixed the legal-info
issues when a custom ATF tarball or a version from git is used.
But we need to ignore licencing for a used defined official ATF version.

Althougt the ATF version are licensed under BSD-3-Clause, the license
file can be updated between version (for example between v1.4 and v2.0).

Ignore the licencing check if the user provide a custom official version.

[1] d1a61703

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: use positive logic with the _LATEST option]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>