dbus-triggerd: new package

dbus-triggerd is a little daemon that can monitor dbus events, match
them against some watch-expressions, then trigger shell-commands.

It can be used in any situation where dbus-interaction is needed, and
shell script is preferred over other languages.

[Thomas: also pass $(TARGET_MAKE_ENV) in the environment.]

Signed-off-by: Arnaud Rébillout <rebillout@syscom.ch>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>