ueventd: add the import option from the init parser

Vendors have an interest in importing ueventd files based on certain
property values.  Instead of baking this logic in the ueventd binary,
add the import option from the init parser to the ueventd parser, to
allow vendors to expand as needed.

Test: imported files are parsed
Change-Id: I674987fd48f3218e4703528c6d905b1afb5fb366