uboot-tools: disable pylibfdt

uboot-tools: disable pylibfdt

The current U-Boot build system assumes that if swig is installed on
the host system, it should build pylibfdt, without checking if other
dependencies are available. This causes a number of build failures.

This commit fixes that by using concepts from the future upstream fix
(post 2017.11), which sets up a kconfig variable for pylibfdt and
conditionally enables the swig wrapper creation (default=n).

Relevant upstream patches under review:

 - https://patchwork.ozlabs.org/patch/826742/
 - https://patchwork.ozlabs.org/patch/826752/

This patch only affects uboot-tools, in which we never need
pylibfdt. The logic in the uboot package, with its
BR2_TARGET_UBOOT_NEEDS_PYLIBFDT option, remains unchanged.

Fixes (one of many):

  http://autobuild.buildroot.net/results/f9e13caae9b31c9dcde7d24caecc036324f269cc

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
[Thomas: improved commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>