feat:[#306] Have option to not format EFI partition

[muhdsalm]

This PR adds an option to keep the EFI partition during install. As of now, the only indication that it works is through VANILLA_FAKE's JSON dump. This has not yet been tested on a Vanilla OS system without VANILLA_FAKE.

This PR works like this:

• The manual partitioning page has a toggle to keep the EFI partition. This toggle sets the fstype of the EFI partition to 'unformatted'.
• The processor skips adding the format option of any partition that has the 'unformatted' fstype. Note that the mountpoint is added before the return statement so that the EFI file can still be added.

I would like to stress again that this has not yet been tested on a live system. If I get the time, I will do it later. Any independant tests would be appreciated to make sure that there are no unexpected surprises.

[mirkobrombin]

Looks good to me, I need a confirm by Mat

[matbme]

I'll test this PR in a VM when I get home just to be sure, but LGTM