package/qt5base: provide "qt.conf" to make "qmake" relocatable

The file "qt.conf" can be used to override the hard-coded paths that are
compiled into the Qt library. We need it to make "qmake" relocatable.

CC: Julien Corjon <corjon.j@ecagroup.com>
CC: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
[Thomas:
 - use | instead of \ as sed separator, suggested by Arnout.
 - fix indentation, suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>