Add support for building TAILS iso from source

[hellais]

It is possible to customise your TAIL image following this guide: https://tails.boum.org/contribute/customize/.

We should add support for building the TAILS image from source and adding some customisation to it (installing for example proprietary BCM43xx drivers that are required by some macbooks).

[cryptomule]

In that case, it would be very helpful for older Macbook's...

• Older models (ex. late macbook pro 2008 4,1) can't boot because of graphic drivers. I believe it's related with the onboard graphic card, NVIDIA GeForce 9400M
• "It appears to be that the official NVIDIA driver doesn't support cards older than the 400 series when booting EFI. Theres some talk about it, but it boils down to using legacy mode of the driver, which has some drawbacks, or to try nouveau and see what happens. This really sucks, but is there any way to make the custom EFI to work with the minimal install images? In this case the driver wouldn't be needed for the install then could be (hopefully) installed in legacy mode. I might try a grub shell boot and test out some options to see what happens :)"