uboot-tools: use local libfdt.h

The use of libfdt.h by u-boot is using the
<system-path>.  The libfdt part of uboot
is expriencing a lot of change and this patch
should be overcome by events post uboot-2017.11
release.

Jan submitted the following upstream
https://patchwork.ozlabs.org/patch/833760/

Resolves:
http://autobuild.buildroot.net/results/347cde4b5c0e6ca76d354396385be4ec1294da73
http://autobuild.buildroot.net/results/2b429f82bed1f03d69021e0f959da4e2db2b5a0b
http://autobuild.buildroot.net/results/b18012963926682ba192b9176bd3e35fb07cdf56



CC: Jan Kundrát <jan.kundrat@cesnet.cz>
CC: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>