I also have that phone and it had fastboot disabled. To quickly answer your question, I haven't found a method to install a fastboot ROM from Ubuntu but it is possible to flash a custom ROM from it.
Assuming there is fastboot ROM installed, you should first boot it into fastboot mode (if you have ADB running, just use
adb reboot bootloader):
- turn the phone off
- hold the camera button and then the power on button
- keep pressing the camera button until you see the LG logo
The phone will be in fastboot mode when you see a blank screen (just the back light will be on). After that, you should be able to use Ubuntu to flash a custom ROM. All you have to do is open a terminal in the folder containing the extracted files and type:
fastboot -w #this wipes user data! don't forget to backup :)
fastboot flash boot boot.img
fastboot flash system system.img
fastboot flash recovery recovery.img
This process should take about 10 minutes to complete.
Just for the record, I downloaded
fastboot binary from here and Swiftdroid 2.0 from here, but I believe you can use any fastboot-ready ROM (in other words, ROMs with
system.img). If you don't have a fastboot-enabled ROM installed already, you can do it using the KDZ Updater as described here, but you have to use Windows and the fastboot ROM.
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