The Zynq ZC706 defconfig does not build anymore since commit
6cda724efb20682bb98e6d738e5f7c909415ae07 ("package/gcc: switch to gcc
6.x as the default"). Fix by upgrading to the latest U-Boot version,
xilinx-v2017.1, based on mainline v2017.01.
In file included from include/linux/compiler.h:54:0,
include/linux/compiler-gcc.h:114:30: fatal error: linux/compiler-gcc6.h: No such file or directory