DON'T MERGE: fix building of U-Boot 2014.04

Something significant has changed so now to build even host tools we need to
config U-Boot for any existing target.

As soon as U-Boot version in Buildroot will be bumpd in upstream this issue
will be somehow fixed.

For now we just explicitly run "make axs101_config" and then safely build tools.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>