Enable thumb build to reduce build size of u-boot.kwb.
Without this, u-boot.kwb overlaps where the u-boot environment is
stored, and updating the environment can break u-boot and vice versa.
https://bugs.debian.org/897671
https://lists.denx.de/pipermail/u-boot/2018-May/327497.html
Signed-off-by: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Stefan Roese <sr@denx.de>
Vagrant Cascadian