Disable file injection in baremetal by default

DocImpact: This disables file injection by default for baremetal. File
injection has never been very useful for baremetal given the much
wider set of device configurations vs virtual, so it's unlikely anyone
was using this (it cannot be used in a heterogeneous OS image
environment, for instance). We should still document the change
though.

UpgradeImpact

blueprint disable-file-injection-by-default

Change-Id: I60a108ef1f0e46ad4da94d2a730426e255011476
Co-Authored-By: Steve Kowalik <steven@wedontsleep.org>
Merge "Disable file injection in baremetal by default"