Replaces 'inherit task' with 'inherit packagegroup' in openoam-enigma2.bb

According to Yocto project documentation, task.bbclass is deprecated in
favour of packagegroup.bbclass.
(https://www.yoctoproject.org/docs/current/ref-manual/ref-manual.html)

Building openoam-enigma2 with Freescale's BSP, newest commit to the
"dora" branch, will fail with an error complaining about the use
of task.bbclass.