Treblize init.rc location

Treblization requires to locate partner-specific modules in its own
partition. So their own init.rc file could be located in /odm or
/vendor.
This CL is to support those locations for the init.rc.
Additionally thic CL modified import parser to support importing a
relative path.

Test: building succeeded and tested on sailfish with enabling early
mount.
Bug: 35269867

Change-Id: I1bce924a32c8a2b53fb5d981d35d758cf9ddd9a6