[Build] make it possible to set jsmn root
[Build] make it possible to set jsmn root

For some build systems like Yocto it is preferable to set the url to a
separately downloaded jsmn.

Change-Id: Id474617f3650fa2eb004dfb57f24d423b682afc4

bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr