Fix issues with paths when using windows

[pimterry]

I know we're moving away from this JS implementation soon, but it'd be very useful to get this in in the short-term anyway as a quick fix to unblock balena-io-modules/balena-device-init#25, where this currently breaks OS configuration in Windows.

Right now, reconfix breaks on windows because it tries to pass paths like \\system-connections\\resin-wifi to resin-image-fs, which is having none of it, and claims that file doesn't exist.

That is the right OS-local separator, but it completely doesn't work for resin-image-fs (as far as I can tell, that expects / all the time). This PR changes this to always generate paths like /system-connections/resin-wifi instead, which always works, regardless of the local platform.

[pimterry]

Ooooh, fascinating: while this fixes the resin-image-fs integration tests that use reconfix, for me on windows, it seems to break reconfix's own tests here on windows. I'll dig into that now to get appveyor passing here (marked as WIP in the meantime). This'll be fun!

I'm going to ignore the circleci error entirely, since it's not a required status, and there's no circle config on this branch, shout if that's wrong.

[pimterry]

In fact, this reduced the number of tests broken (master currently fails even worse on windows), and the remaining issues were similar windows path.join bugs in the test code itself. Now fixed too, we should get everything (ignoring circle) merrily passing now 🤞

[jviotti]

@pimterry Great findings, I'll get you a version on npm in a minute. @abrodersen is this something we should keep an eye on on the Rust fork?

[jviotti]

@pimterry Check v0.0.4 on npm!

[pimterry]

@jviotti perfect, thanks!