gpsd: adjust udev rules file permissions

Adjust a generated udev rule file (25-gpsd.rules) to have user write
permissions. This is to prevent rebuild issues when the gpsd package
fails to re-install on a target (since the `cp` of the rule file will
fail due to permissions).

[Thomas: minor tweaks to code comments and commit title.]

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>