rootdir: add pstore

/dev/pmsg0 used to record the Android log messages, then
on reboot /sys/fs/pstore/pmsg-ramoops-0 provides a means
to pull and triage user-space activities leading
up to a panic. A companion to the pstore console logs.

Change-Id: Id92cacb8a30339ae10b8bf9e5d46bb0bd4a284c4