bitbake: toaster: implement BitbakeController.getVariable

Added new API to get value of bitbake variable from bitbake server.
The API will be used to update INHERIT variable instead of writing
it to the toaster configuration files.

(Bitbake rev: effc371e3a4a2262e53e9e560c7c08cffad195da)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/toaster/bldcontrol/bbcontroller.py

@@ -52,6 +52,9 @@ class BitbakeController(object):
     def setVariable(self, name, value):
         return self._runCommand(["setVariable", name, value])
 
+    def getVariable(self, name):
+        return self._runCommand(["getVariable", name])
+
     def build(self, targets, task = None):
         if task is None:
             task = "build"