Add --all option

[chrysn]

This option applies the requested operation at all connected bootloaders.

There's one use case I think for which this is really practical: listing the devices using --get-image or --all.

When --all is given, the device (and serial number) is shown in front of all items, so you might get:

$ nrfdfu --all
Port /dev/ttyACM2 (serial: "EB86723B4658")
Port /dev/ttyACM3 (serial: "D2C505A7E0C6")
error: No actions performed; provide an .elf file on the command line to flash, or set querying options.
$ nrfdfu --all --get-images
Port /dev/ttyACM2 (serial: "EB86723B4658")
* image 0: Bootloader, version 0. Starting 0xe0000, length 122880.
* image 1: Application, version 0. Starting 0x1000, length 9412.
Port /dev/ttyACM3 (serial: "D2C505A7E0C6")
* image 0: Bootloader, version 0. Starting 0xe0000, length 122880.
* image 1: Application, version 0. Starting 0x1000, length 19556.

I'm not sure there are use cases for all combinations, but they kind of fall out neatly from how things are set up -- so you can also flash a file to all connected chips (convenient device-to-device tests) or reboot them all into the firmware. The most useful application IMO is getting the serial numbers for later use with --serial -- that wouldn't need --all fundamentally, but it fits nicely. I'm open to other ideas just as well (eg. not making it too --allish and instead having a --list option that behaves largely like --all now standalone, but only combines with --get-images). Or anything else that makes this better user-experience-wise.

[jonathanpallant]

Looks good to me - just one query about the phrasing.

[chrysn]

Thanks for the review, I've applied the suggestion.

(Also, I don't have anything immediate in the pipeline -- I do want to have a look at #17 at some point, but that'll have to wait a bit. So if the current activity around this package, which I think was caused by people looking into using nrf52840dongle for Ariel OS, has otherwise subsided, after this might be a good time for a release if you could manage.)

[jonathanpallant]

Perfect timing. I have a training starting Wednesday.