meta-agl-core: use next branch for weston dependent packages

meta-agl-core: use next branch for weston dependent packages

Add a "aglnext" OVERRIDE in the distro configuration, and use it
to select the "next" branches in the agl-compositor and
waltham-transmitter-plugin recipes, and use AUTOREV for their
SRCREV.  The latter will allow rebasing the next branches in
those repositories as next branch updates occur without requiring
SRCREV updates every time.

Bug-AGL: SPEC-3819

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I4f2a57fface53374d9428b5af1c6ad02edfde7eb