Enable build with MingW (using Cygwin to create configs).

1. Create configs on Cygwin like this: make ARCH=arm vexpress-a15-defconfig
2. Build image on MingW like this: make;
3. If the dtc.exe or kallsyms.exe are not there, go to tools/dtc/ or tools/kallsyms/, issue make and copy the dtc.exe and kallsyms.exe into build/tools directory.