heimdall-frontend: Make the OUTPUTDIR variable work properly

$$OUTPUTDIR and $$(OUTPUTDIR) are distict; the former is a config
variable and the latter an environment variable, but
heimdall-frontend.pro mixes them. To get useful output
the user would have to invoke qmake like this:

OUTPUTDIR=/usr/bin qmake OUTPUTDIR=/usr/bin

This changeset allows the user to specify OUTPUTDIR via the environment OR
on the command line.