build: no longer build emulator by default

This is just increasing build times when most people are not using it.
Let's speed up builds a bit.

Building emulator should still be possible by appending BUILD_EMULATOR=true
to the build command

PS2: Let's keep the darwin warning.

Change-Id: Ibd8e2b90455bd603a691c7b95466068b6fae212a
Signed-off-by: Andrew Warfield <hiasant@gmail.com>
build:  do not show TARGET_BUILD_APPS unless it is set

https://github.com/codefireXperiment/build/commit/203c86c0ea6073b967f4d31f076824ae4adb8bc5

Change-Id: Iaeb8899d54a027986f264a610fd485654565dfa7
Signed-off-by: Andrew Warfield <hiasant@gmail.com>