fs_mgr: support reading fstab based on ro.boot.hardware.platform

This change adds fstab.${ro.boot.hardware.platform} file into list of
searched fstab filenames.
The ${ro.boot.hardware.platform} helps consolidate init sequence based
on platform. Potentially it allows same build serve different devices.

Bug: 68663168
Change-Id: I822d5bc432e823218ebc640093cfbe5e9271d3b1