ubuntu-device-flash should give options to select bootstrapping method for touch devices.

Currently, the only bootstrapping method provided by ubuntu-device-flash is via fastboot. As I port Ubuntu Touch to a few devices, I discovered that fastboot isn't always (easily) available.
  * Samsung devices don't have fastboot. Instead, they provide download mode which has similar abilities to fastboot.
  * LG devices have fastboot, but one must wipe the boot partition to trigger it. LG devices also provide download mode (although not the same one as Samsung devices), with similar abilities.

So, I think ubuntu-device-flash should let users choose how their devices are bootstrapped. Possible choices are:
  * Fastboot
  * ADB (Useful when one is running other system or recovery and doesn't know how to reboot to fastboot or their bootloader mode isn't supported by this tool yet.)
  * Samsung's download mode (using heimdall, available in universe)
  * LG's download (using lglaf, available at (https://github.com/Lekensteyn/lglaf/). May have to be packaged.)