dtbtool: fix for filesystems that don't return d_type from readdir

readdir() doesn't return d_type for all filesystems, in particular XFS
doesn't fill in this field. fix by using stat() instead of d_type to
determine if a dtb is a regular file. code is also rearranged slightly
to avoid doing a redundant stat().

Change-Id: I26b402f517cf8ecefa78d8ae45e51219b333b682