On some systems the host system or even the cross sysroot can
contain a version.h. This leads to the wrong file being picked
up and a PLAIN_VERSION undefined error.

This workaround symlinks the version.h into the tool folder to
allow reordering of search folders.

Fixes
http://autobuild.buildroot.net/results/770/7702d5df36a6532aafdbe6e9e62709bbfa058b54/build-end.log
http://autobuild.buildroot.net/results/e34/e3401027d2fb3ce565ca9e2456a427afd3610a87/build-end.log
... additional can be found with these queries ...
http://autobuild.buildroot.net/?reason=uboot-tools-2018.03
http://autobuild.buildroot.net/?reason=uboot-tools-2018.05

Upstream: pending

Signed-off-by: Matthew Weber