image_types.bbclass: Populate cpio /init as symlink

image_types.bbclass: Populate cpio /init as symlink

For cpio/ramfs the kernel will first attempt to execute /init and will
emit the following error as the file is empty:

    Failed to execute /init (error -13)

If /sbin/init exists symlink to it so the kernel can immediately start
the correct init executable instead of an empty file.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>