Automate source ramdisk selection

This commit automates the process of selecting
the relative path for custom kernels

To make this change apply on your device include

TARGET_KERNEL_CUSTOM_RAMDISK := true

in you BoardConfig in device or common folder.

Thanks to Manfred for the idea!

Change-Id: I0e6bb148097120a828fecd4427e863787d944915
Merge "Automate source ramdisk selection" into jb4.2