Add possibility for extra qmake args in bld_app.py

Allow extra args for qmake when in bld_app.py to be able perform e.g.:
qmake CONFIG+="debug_and_release build_all"

Change-Id: I3890be1e8e1b692f93ca27c398d951c18aea0080
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>