Update to Yocto Project 3.1.2 release

Update to Yocto Project 3.1.2 release

Changes:
- Update various layers to latest dunfell branch revisions to match
  Yocto Project 3.1.2 release.  Note that the poky commit used
  includes some security fixes after the tagged 3.1.2 commit.
- meta-clang now has a dunfell branch, so its configuration has been
  updated to use it.
- meta-ti has been left at its latest release tag (07.00.00.005) to
  avoid updating meta-arm and picking up its new (hopefully temporary)
  meta-arm-toolchain dependency.

Bug-AGL: SPEC-3529

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I662bba42bebf08d287e463c6f72b02cccde3e6ab