The header of LiMon imported files reference a License file which
does not exist in U-Boot. Some files were forgotten when adding the
SPDX-License-Identifier.
The LiMon files were originally licensed under GPLv2 as can be seen in
commit [2ea91039].

Based on this commit, add the correct SPDX license identifier.
While at it drop the reference to the non-existing License file from all
LiMon files and update the SPDX-License-Identifier to SPDX version 3.

Signed-off-by: Max Merchel <Max.Merchel@ew.tq-group.com>

This change gives the ability to reuse the <tftp.h> header file by other
subsystems (like e.g. dfu).

Without this change compilation error emerges for the legacy update.c file.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>