ueventd: Add bootdevice symlink for boot storage device

Create /dev/block/bootdevice symlink for the boot storage
device based on ro.boot.bootdevice property.  The property
value can be passed in via bootloader.  This abstracts
references to the boot device (including partition nodes
in the boot device) from the actual device nodes (e.g. eMMC,
UFS).

Change-Id: I9c921fc9e31855759435fac27bd384645eee0236