eglfs: Pluginize RPi, iMX6 and Mali backends

eglfs does not depend on the device makespecs anymore when it comes to these device
integration backends (hooks). Instead, backends are autodetected by configure.

The name of the preferred plugin is still set in the device makespecs. This
is optional. When not set and there is more than one plugin present in the system,
the environment variable QT_QPA_EGLFS_INTEGRATION will have to be set at runtime.
In the absence of that, the order is undefined.

Change-Id: Ie1ced2c9aa1beff2adb13b4fdea7c499cb5a6aab
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Andy Nichols <andy.nichols@theqtcompany.com>