st/egl: Build a single EGL driver.

st/egl: Build a single EGL driver.

This change makes st/egl build a single egl_gallium.so and multiple
st_<API>.so and pipe_<HW>.so.  When a display is initialized, the
corresponding pipe driver will be loaded.  When a context is created,
the corresponding state tracker will be loaded.

Unlike DRI drivers, no ABI compatibility is maintained.  egl_gallium,
pipe drivers and state trackers should always be distributed as a single
package.  As such, there is only a single src/gallium/targets/egl/ that
builds everything for the package.