Install script support

Copy files from OUT/install to ZIP/install and extract to /tmp/install
at install time.  This is an extensible framework that may be used for
install tasks as needed.  Note that the install files are extracted to
a ramdisk, they are never written to /system.

Change-Id: I9f9e71918c7369b907365b6cb6bc143f2cd8aab8
Variant support for device families

Setting TARGET_VARIANT_SCRIPT to true will cause install/bin/variant
to be packaged and run after /system has been extracted but before
permissions have been set.  The variant executable may do whatever is
necessary to detect and accommodate the specific device model.

Change-Id: Icce450a6d514a37fcef1da8eb4c9f7007cab1c7c