introduce AUX build class of targets

AUX is a new class, similar to TARGET
While TARGET defines toolchain for Application Processors
AUX is defining toolchains for arbitrary utility cores (DSPs, GPUs,
MCUs, etc). This allows building of non-android sources as part
of Android tree and avoid using prebuilts if source code is avaliable

Bug: 29635686
Change-Id: Ie755ea054b16c3e86369f5fb2ba6eb0b384af77f
Signed-off-by: Alexey Polyudov <apolyudov@google.com>