eglfs: Allow using a different framebuffer device

Right now /dev/fb0 is hardcoded. This is not ideal. Therefore
QT_QPA_EGLFS_FB is introduced. This environment variable can be set to
a different framebuffer device. Once it is set, eglfs will use the
specific device. This is similar to linuxfb's fb=... plugin parameter.

The actual behavior depends on the board-specific implementations.
For now only iMX6 has real support. It extracts the index from the
device name as bind the EGL display to the corresponding framebuffer
using the vendor-specific fbGetDisplayByIndex(). Other hooks can
follow suit later on.

With this patch eglfs is at least on par with linuxfb, meaning that,
if the board supports it, different apps can run on different screens.

Task-number: QTBUG-36113
Change-Id: Ia3c88bd06e108bc668433e3c5c3fce34a5a0e73d
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>