Multiple flash methods

Some devices support multiple flash methods by default. Since #404 is merged, you can specify the flash method manually from the command line with the --method option. However it is not safe to allow using every flash method on any device.

The plan is to only allow flash methods, which are enabled in the device's deviceinfo file (deviceinfo_flash_methods).

@ollieparanoid said we should add adb (recovery flashing) to the deviceinfo too, but I disagree, because TWRP is not something installed by default, hence I think it doesn't belong in the deviceinfo file.

We would like to hear the community's opinion on this, so we could fix this.