Rename "/writable" to "/data"

[Gustavo Niemeyer]

(from https://github.com/ubuntu-core/snappy/issues/6)

mvo commented on Oct 22, 2015:

We agreed that /data is a better name for the writable partition at the last sprint. Lets rename it.

[Oliver Grawert (ogra)]

if we ever start supporting snappy ubuntu phones on devices where we can not (or do not want to) re-partition this might potentially become a problem.
usually on these devices the android userspace partitions are named "data" or "userdata". the search algorithm in the initrd uses the partlabel name and will have to become a lot cleverer (and due to this more error prone) to catch this then.
the initial choice of "writable" did work around this potential name clash.

note that a change of the initrd code will most likely also affect boot speed.

[Michael Vogt (mvo)]

This was not renamed because of the concerns about compatibility that ogra raised. Please reopen if we still should try to find a better name than `writable`.