Add support for persistent Linux config files

[wizzomafizzo]

This is a proof of concept as discussed. I've been pretty liberal adding TODO comments that would need to be clarified before it's considered.

The basic idea is:

• User keeps a copy of certain config files they have customised in the linux folder on the SD card
• During linux update, downloader checks if these files exist
• If they exist, after the linux.img file has been copied over and update is finalised, mount the .img and copy the user files into it

Simple enough.

I've picked out these files:

• /etc/hostname
• /etc/hosts
• /etc/network/interfaces
• /etc/resolv.conf
• /etc/dhcpd.conf
• /etc/fstab

I think the risk here is very low, as it's purely opt in. There's a question of whether or not to handle official updates to these files. Notify the user? Attempt to merge? Ignore it? I'm leaning with ignore it personally, as the other options add a lot of complexity. I can't imagine these files will change much anyway.

I don't know how this change might affect your PC downloader app if at all.

[wizzomafizzo]

Feedback is now incorporated. I'll give it a spin tomorrow and make sure it actually works :)

[theypsilon]

@wizzomafizzo I've tested your changes, and everything worked as expected!

I'd want to introduce a small change on top of this. I think is a bit more safe mounting the Linux image when it's at /media/fat/linux.update/files/linux/linux.img and aborting the linux update altogether if there was any error on the _restore_user_files while using the linux.update folder (_restore_user_files now should return a bool for checking this). What do you think?

[wizzomafizzo]

I'd want to introduce a small change on top of this. I think is a bit more safe mounting the Linux image when it's at /media/fat/linux.update/files/linux/linux.img and aborting the linux update altogether if there was any error on the _restore_user_files while using the linux.update folder (_restore_user_files now should return a bool for checking this). What do you think?

Makes sense to me, I agree. Do you think the method should be called just before the "Hold your breath" print?

[theypsilon]

Yes, right there, it's the best place.

[wizzomafizzo]

I haven't tested yet but I think that should do it :)

Seems pretty tight to me now. Every step of the process is error checked and aborts the process now except creating the temp dir which I assume will make the mount fail anyway.