Changed ebuild to work with recent version.

Drop existing Makefile patch which does not work anymore and looks
unneded.
Added short text description about installation.
Modified ebuild to correctly configure for selected board.
Optional 'opengl' IUSE for mesa and xorg-server

Added possibility to install media-libs/mesa and x11-base/xorg-server
without libGL because it does not work properly anyway with binary
driver.

media-libs/mesa is installed with .gles-only file which is understood by
recent enough version of eselect-opengl.
TODO: x11-libs/sunxi-mali should also behave in this way.

Almost everything is copypasted from
git:ssvb/gentoo-overlay-mali-x11 which is a little old with respect to
current stable portage versions.