Iris Gallium driver of MESA for Intel GPUs

Genode specific option:

In order to improve performance the Intel driver implements a buffer-object
cache which is quite memory hungry. In order to safe memory and lower
performance the following environment variable can be set in a component's
"config" node:

XML:

! <env key="IRIS_USE_BO_CACHE" value="false"/>

HID:

! + env key: IRIS_USE_BO_CACHE | value: false
