emulator: Remove the global Make variable trick for emulator-specific system modules.

Remove a sad trick that was used to smoothly move the platform-specific emulator
modules from sdk/emulator/ to development/tools/emulator/system without creating
build conflicts.

Now that the sdk/ modules have been removed, we can get rid of the guard variable.

Change-Id: I0261fcd6cdf6af7564106c5ab8d2b3bda001d567
4 files changed