Make root readonly, create a partition for persistent data

[nisargjhaveri]

In an attempt to reduce SD card failures due to power outages when being written:

• Create a new partition and mount it at /persist. Use this partition for things we need to persist like the bluetooth connection info.
• Make root readonly, reducing the chances of unnecessary writes to the sd card.

[nisargjhaveri]

This should help with #138 and #109. This seems to reduce SD card writes. It only writes on boot or when bluetooth is connected. Would be great if more people can try out.

Not sure what else was continuously writing on root though. Logs are anyway on tmpfs. But this looks like a good idea anyway to reduce chances of some service accidentally writing.

[pavitra14]

Flashed this today, Let's see how it goes!
Thanks @nisargjhaveri !

Update: Haven't faced any issues with the card anymore.

[BaderSZ]

While we're at it, could you also make /var/log link to /persist/log?

Ideally, this would only be the case for 'development' images, but since those aren't released, it should be fine as is. The question is what to do if the partition is full (BR2_PACKAGE_LOGROTATE?)

[nisargjhaveri]

@BaderSZ having logs writing to sd card will increase the chances of sd card corruption by a large margin. I don't think it is wise to persist logs by default.

Having separate development images is a good idea, or I was maybe thinking of having a config that can enable logs to persist.

[BaderSZ]

@nisargjhaveri that is true, hence my suggestion for the dev image as a workaround, as opposed to people building their own. Alternatively, writing the logfile at set intervals to the SD card is also an option, but I am not aware of an easy way to do that with buildroot.