Support dynamic partitions feature by default

Build with dynamic partitions enabled by default, which produces a
"super.img" file instead of the {system,product,vendor}.img files
generated previously. The system image is no longer a "root image"; it
still contains a useful root filesystem, but the true root is now
contained in the ramdisk, which chain loads it after the dynamic
partition has been constructed.

Use TARGET_USE_DYNAMIC_PARTITIONS=false to disable this feature.

Bug: 113175337
Change-Id: I15e951e6bf7d9bfced33c5266b3c8058fd544950
Merged-In: I15e951e6bf7d9bfced33c5266b3c8058fd544950
Signed-off-by: Alistair Strachan <astrachan@google.com>