This experiment tries to make a partition table inside a partition using a loopback device.
This makes it possible to sub-partition a "system" partition into "system" and "vendor"
and even "oem" "odm" "factory", or whatever can be useful.

Steps:
- Apply platform_system_core.diff to system/core
- Apply platform_system_sepolicy.diff to system/policy #No neverallow harmed, so certifiable!
- Create your new system partition:
$ bash buildSystemGpt.sh system /where/is/your/original/system.img vendor /where/is/your/vendor.img
- Flash the generated sytem-gpt as "system" partition (like fastboot flash system system-gpt)
- Change dts, example:

>   firmware: firmware {
>           android {
>                   compatible = "android,firmware";
>                   fstab {
>                           compatible = "android,fstab";
>                           loop {
>                                   compatible = "android,loop";
>                                   dev = "/dev/block/platform/soc/7824900.sdhci/by-name/system";
>                                   type = "loop";
>                                   mnt_flags = "default";
>                                   fsmgr_flags = "default";
>                                   status = "ok";
>                           };
>                           vendor {
>                                   compatible = "android,vendor";
>                                   dev = "/dev/block/loop/loop/by-name/vendor";
>                                   type = "ext4";
>                                   mnt_flags = "ro,barrier=1";
>                                   fsmgr_flags = "wait";
>                                   status = "ok";
>                           };
>                           system {
>                                   compatible = "android,system";
>                                   dev = "/dev/block/loop/loop/by-name/system";
>                                   type = "ext4";
>                                   mnt_flags = "ro,barrier=1";
>                                   fsmgr_flags = "wait";
>                                   status = "ok";
>                           };
>
>                   };
>           };
>   };

Notes:
- example here lists only system/vendor, but it applies for any partition, like future odm/oem/factory partitions
- The changes are quite horrible at the moment, but it's okay for google certification
- This has been tested on an A-only device, with AOSP 8.1 system.img, CAF 8.0 vendor.img
- platform_system_core.diff will change boot.img
- platform_system_sepolicy.diff will change vendor.img

TODO:
- recovery support not implemented yet