Bug 931718 - Introduce a 'print-xulrunner-sdk' make target

In order to be able to run mochitest-remote, for example on emulator, we
need to have a way to get the correct path of the XULRunner SDK.
Merge pull request #13217 from lissyx/bug931718-v2-gaia

Bug 931718 - Introduce a 'print-xulrunner-sdk' make target