Port to POSIX

[trn1ty]

It does the same thing, better, in 75% of the code, and is compatible with more shells. However I haven't tested it. You should probably test it before accepting the PR. I did this while very tired so apologies if I made a silly mistake.

[NeilInnes]

Thanks - on a quick glance this looks good, maybe needs a bit more output so keep the user informed. I'll give this a good check over 👍

[trn1ty]

It has about as much output as before. set -e makes it terminate on error, and the various ls >/dev/null constructs are silent on success but report an error on failure. This also means most of the localization is provided by the operating system.

Much of the debugging messages were communicating things already known to the user or implied by the function of the program, so where things weren't obvious I pushed the information to stderr. The biggest spot of information is in dancing around finding the bootloader path, but the situation where someone has a uConsole OS without proper bootloader directories is rather unlikely, and the error message would still be helpful to them by showing them the path.