bmc: only build file handlers if configured
bmc: only build file handlers if configured

The file handlers for the various configurations were built but only
used when appropriate.  Do not build them if they're going to be
presumably stripped out during linking.

Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>
Change-Id: I16765bb263b3ecefb12d592bbd06b954663475ff

openbmc/phosphor-ipmi-flash