default-providers: Add VIRTUAL-RUNTIME_dbus variable

default-providers: Add VIRTUAL-RUNTIME_dbus variable

The purpose of the new variable is to facilitate oe-core users who wish
to use an alternative runtime D-Bus implementation instead of the
default Freedesktop.org's dbus, such as dbus-broker, a recipe for which
is currently available under
meta-openembedded/meta-oe/recipes-core/dbus/

While introducing this facilitation the intent is to preserve the
existing functionality, while allowing the user to optionally select an
alternative runtime D-Bus implementation by adding the following line
e.g. to local.conf file:

  VIRTUAL-RUNTIME_dbus = "dbus-broker"

As a background, for example the Fedora distribution uses dbus-broker
instead of Freedesktop.org's D-Bus implementation. The following
excerpts from
https://fedoraproject.org/wiki/Changes/DbusBrokerAsTheDefaultDbusImplementation
provide background for their technological decision

  This change provides a more scalable and more reliable implementation in place of the reference implementation.
  The reference implementation suffers from long-standing issues including potential dead-locks and susceptibility to denial of service attacks, which the replacement implementation does not.

and

  No visible changes in behavior are expected, except for different log messages.
  Any visibly different behavior to `dbus-daemon` should be reported as a regression.
  Ideally, this change should only improve the performance and security of the message bus.

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>