Use Raspberry Pi Linux's ramoops overlay

[fhunleth]

Upstream Linux just recently received a patch to add a ramoops overlay for all of the Raspberry Pis. If we switch to use it, then we can stop maintaining our own ramoops overlay.

See https://github.com/raspberrypi/firmware/commit/cbcefbc87a72458406c924f44d2525fe76470efb.

It's also in the latest rpi firmware package, but it is newer than the one we're using right now.