* Add support for RPI4 and generic KMS targets
* Allow configuration of rpi3 platform in favour of rpi2 platform
* Move build configuration into separate function to avoid duplication between
build and install functions.
* avoid duplication of addEmulator definitions due to audio backend options
* Set alsa as default audio backend for mesa (omx doesn't work, and oss backend via
aoss causes emulator stuttering).
Patch details (may be removed after upstream is fixed):
* Add SDL/gles2 sub-platform for rpi4 to allow kmsdrm compatibility
* Ensure vsync is enforced for SDL, otherwise KMSDRM will run at unlocked framerate.
* Fix signal handler conflict by disabling SDL2's exit signal handler, and ensuring that
reicast gracefully cleans up by stopping emulation and invoking SDL_Quit() during exit.
