Introduce Squisher (1/2)

I've made this option optional obviously because of the controversy involved with Opticharging/Squishing ROMs.

I recommend you install pngquant because it does the best job of shrinking PNGs and I don't notice any difference between full size and quantized PNGs. I see about ~7-8 MB change in zip size on my Nexus 5 when using this option.

To opt in use SQUISH_BUILD := true to your BoardConfig.mk. You'll see bright green near the end of you build letting you know the Squishing is taking place.

Signed-off-by: Chet Kener <Cl3Kener@gmail.com>