OpenWrt 18.06 build system port to cygwin

[lexa2]

@neheb

seem to be perfectly fine.

An extra bug fix is required if you're building squashfs-based image. Here are details:

[solved] Strange mksquashfs4 issue

So I nailed it. First I tracked it down to be openwrt-specific as if I build squashfs tools without openwrt patches mksquashfs create byte-to-byte identical FS archives on cygwin and linux if fed with the same source set of files. Then I applied OpenWrt-supplied patches and compared the differences in the images created under cygwin vs linux. The only differences were in the 12 bytes header known as COMPRESSOR OPTIONS. Rest of the file was bit-to-bit identical. Looking and headers differences i…

Hadn't pushed this change to my github page yet.