always rebuild build.prop

Change-Id: I9a6f4706cdc3e0ca4e40acb1b00419d72a236f65
Bring back squisher as an optional part of the build

Some devices could benefit from the size reduction from running
squisher, so bring it back as an optional part of the build process.

Change-Id: I288f694fd1bf26782e05dcfa797a0569d9fdf9cb