bitbake: toaster: libtoaster Update implementation of startABuild and cancelABuild

Update the implementation of startABuild and cancelAbuild to reflect
changes to the backend api. We now have a dedicated endpoint to make
calls into so add this url to libtoaster.ctx and allow passing null in
as a url value to indicate that we want to use the current project

Also:
 - Fix some documentation comments
 - Add the convenience of passing in an array of targets to startABuild

(Bitbake rev: 61a21d96abab113cbd13376cdb8b08a426b50538)

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/toaster/toastergui/templates/base.html

@@ -47,6 +47,7 @@
         projectBuildsUrl: {% url 'projectbuilds' project.id as pburl %}{{pburl|json}},
         xhrCustomRecipeUrl : "{% url 'xhr_customrecipe' %}",
         projectId : {{project.id}},
+        xhrBuildRequestUrl: "{% url 'xhr_buildrequest' project.id %}",
         {% else %}
         projectId : undefined,
         projectPageUrl : undefined,