buildroot

uboot-tools: disable libfdt swig wrapper for host

Commit f4891c398e599f18bbf41eb33885930431f5e1c8 [1] added a workaround to
disable the build of the python libfdt module if swig is available on the
host.

This workaround is also necessary when building the host uboot-tools.

Note, that the issue was introduced in upstream commit
1905c8fc711a527ff10550425498bc77e4db9ac3 [2] and released in version
U-Boot version 2017.03 and fixed in version 2017.07 (see [3]).

[1] https://git.busybox.net/buildroot/commit/package/uboot-tools?h=master&id=f4891c398e599f18bbf41eb33885930431f5e1c8
[2] http://git.denx.de/?p=u-boot.git;a=commit;h=1905c8fc711a527ff10550425498bc77e4db9ac3
[3] http://patchwork.ozlabs.org/patch/787412/

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Acked-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>