uboot: use local libfdt.h

Use of libfdt.h by u-boot is currently using
the <system-path>.

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

Resolves a failure like this one observed on uboot-tools
http://autobuild.buildroot.net/results/347cde4b5c0e6ca76d354396385be4ec1294da73



[Peter: only sed if file exists]
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>