[Solved]emulator: ERROR: You did not specify a virtual device name, and the system directory could not be found.

Problem: After building AOSP and trying to lunch the emulator, coming up with this error:

               emulator: ERROR: You did not specify a virtual device name, and the system directory could not be                            found. 

               If you are an Android SDK user, please use '@<name>' or '-avd <name>' 

               to start a given virtual device (see -help-avd for details). 

               Otherwise, follow the instructions in -help-disk-images to start the 

               emulator 

Solution: type ./emulator -help-disk-images, then you would know how to fix it.