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.
Merge remote-tracking branch 'upstream/master'

this merge is necessary, for next sunxi-mali update.
USE-flag to select latest r3p2 git-branch.

With latest kernel driver from ssvb repository it is possible to use
both cores of A20 CPU present on Cubieboard2. This commit allows to
download corresponding branch for xorg driver right now before its code
is submitted to master-branch.
Rewritten ebuild for mali binary driver.

Changed the way installation is done. Provided Makefile is used only for
headers installation, while libraries are installed manually by ebuild.

Also compiles and installs provided EGL test executable.
Merge pull request #2 from spellbuilder/master

Bunch of updates.