Documentation updates for Raspberry Pi

[Hodapp87]

The documentation for Pi 2 and Pi 3 says to use es2-rpi for CINDER_TARGET_GL, but given https://github.com/cinder/Cinder/blob/master/proj/cmake/configure.cmake#L83 this looks to be out-of-date and it triggers the "Unexpected value for CINDER_TARGET_GL" error. The error message itself also claims es2-rpi is valid when it's not.

I was running this on Raspbian Buster and also ran into a few missing X-related dependencies which I added into the documentation. I suppose this applies identically on rpi2.html but I'll leave it up to you whether that needs updating or not.