Allow some of the firmwares to be built on GNU/Linux and other UNIX-like platforms

The Windows path separator `\` has been replaced with `/` in firmware source files.  Firmware should continue to build successfully on Windows, but I will need someone to test that for me.

Additionally, I've added a bash equivalent of the `make_all.bat` script for convenience.

I say "some of the firmwares" because on GCC 4.8.2 there are other build errors with most of the firmwares, but they aren't because of platform-specific code.