aarch64: Use console=ttyAMA0 instead of console=ttyS0.

This still only prints early console messages.  Output stops
after:

  bootconsole [earlycon0] disabled

but I have not found any way to get later kernel messages.
aarch64: Add some earlyprintk debugging.
ocaml: Ignore Warning 3: deprecated feature: String.* functions.

In OCaml 4.02, the 'string' type can be made immutable.  All String.*
functions that are concerned with creating or mutating strings now
give a warning like this:

  Warning 3: deprecated feature: String.unsafe_set

Disable this warning since we don't want to turn on immutable strings
yet.