Add support for exec command in init scripts.

Change-Id: Iee523fd75410718f0bd090f5fdcba50825a698c4
init: Add support for properties expansion in exec command

Add Android properties expansion to 'exec' arguments.
E.g.  exec /system/bin/log ${sys.usb.config}

CRs-Fixed: 390942
Change-Id: Ie520396d951e578bd9f591734a45d2f460b8f51d