support for ext4/EMMC in target_files and OTA generation

Move the image sizes into a more generic key-value file.  Make them
optional.  Add additional key/value pairs describing what kind of
filesystem the device uses.  Pass new fs-type-related arguments in
edify scripts when mounting and reformatting partitions.

Don't include all the init.*.rc files from the regular system in
recovery -- they aren't needed, and break recovery on some devices.

Change-Id: Ic1c651f754ed00ba1cffe8cf56c43f7f3b0ebfd7