Search fstab also from /first_stage_ramdisk

I828ce999be6d786bf46dd5655dfda81d046906ab made a slight behaviral change
that fstab is searched and read before /first_stage_ramdisk is mounted
as root. Without this change, the attempt to read fstab from / fails at
the moment, leaving an empty fstab object. But as a side effect of the
attempt, DoCreateDevices() is not called again even after
/first_stage_ramdisk is mounted as root and the fstab is found under /.

This change fixes the problem by adding /first_stage_ramdisk to the list
of places to find the fstab file.

Bug: N/A
Test: Watch TH
Change-Id: I9826610cce436ba706aaea14c9a991822d2bae96