Need a way to get a list of all the configs

[danielpieczko]

With xcommon, you could run xmake allconfigs and it would list all the application configs. This is used by the sw_usb_audio test system to auto-generate the list of configs to test. There could also be use-cases where a script needs access to the list of configs. The configs are all printed when CMake configuration is run. Not sure how this can be fit into the two-step CMake process. Maybe we always create a custom target called "allconfigs" which prints the list of configs.