ueventd: Add bootdevice symlink

Create the /dev/block/bootdevice symlink to point to the
boot device information based on kernel command line.
This decouples references to the boot device from the
actual device nodes (e.g. eMMC, UFS).

Change-Id: If54b0b6ba6007742e9cb3d9d90cb3c86cd30b898
IMS-VT: Define IMS group.

Define IMS group.

Change-Id: I548277606b0fb5cedc0aa5c7eca0147088bf9da7
CRs-Fixed: 615689