init: allow init file imports to use properties in names

Adds new property syntax in init files during init file filename
expansion during the import command:
${prop.name}

So, one can do:  import /init.${ro.hardware}.usb.rc

Should convert other usages of property names to use the new function.

Change-Id: I9205d7d7a2da620bc8e6b89ac0eb554fad53ded3
Signed-off-by: Dima Zavin <dima@android.com>