init: allow kernel command-line for mount

Replace $BD encountered in fstab with android.bootdev kernel cmdline
property to allow a flexible boot device.

https://github.com/boundarydevices/android_system_core/commit/c3136fff4ede00b113
94ed42bb52dbb9b3af670e

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>