build: Crush all the PNGs

Refactored CM's opticharger to match with MoKee's build system,
will only optimise UNOFFICIAL, NIGHTLY and EXPERIMENTAL builds

Change-Id: I8f2948fad5711d2eadc376372cbec6e21e0ab509
build: Crush RELEASE builds too

Thanks Google for retaining classes.dex in odexed APKs since KitKat...
thank you soooo much for making odexed builds so large...

But with that, RELEASE builds are safe for crushing too...lol

(╯°□°）╯︵ ┻━┻)

Change-Id: I3c248a486c85c37c1deb3d49279cafbd44995f00
build: Do not fail if no tools (for pngcrusher) available

And put a warning if pngquant is not installed

Change-Id: If1ce2eac8fbe2de48dba0bf78ba318306070565f

Conflicts:
	envsetup.sh