qt5_tool: Remove configuration file key developerBuild.

The key written to the configuration file was determined from the
initial user input and then no longer used.
Add -developer-build automatically unless specified or a prefix
is given.

Change-Id: I6dda75d2a92bdf79e1bbe31e72fda6af657bd87f
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
qt5_tool: Fix configuration file help.

Change-Id: Ifacfc9ccf6ac7c967d1aae4f52801865bf108a43
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>