lavd/fbdev_enc: add default device

Use device specified by environment variable FRAMEBUFFER or /dev/fb0 as default.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Merge remote-tracking branch 'lukaszmluki/master'

* lukaszmluki/master:
  lavd/fbdev_enc: add default device

Merged-by: Michael Niedermayer <michaelni@gmx.at>