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>