Add support for data_subdir kernel cmdline argument

* Enables having the system in subdir of the /data
  partition, useful when multibooting
* This modification is currently used by MultiROM,
  a multiboot modification for several Nexus
  devices.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
Merge pull request #33 from Tasssadar/hybris-10.1

Add support for data_subdir kernel cmdline argument