Add checksum to compiler build process

[EmbeddedMan]

To package the compiler for Arduino 1.6.6 consumption, a checksum is needed on the compiler. While chipKIT-core can build this checksum every time the core is built, the best thing would be for the compiler build process to include the creation of this checksum.

See chipKIT32/chipKIT-core#108

[jasonkajita]

@mariusgeanta Added *.sha256 files to the release https://github.com/chipKIT32/chipKIT-compiler-builds/releases Will this work, @ricklon?

[EmbeddedMan]

The .sha256 files do work. It would be easier if the file JUST contained the SHA256 hash, rather than the hash and the filename. But we can probably work around that.