checkfc: do not die on 0 length fc's

Checkfc was treating 0 size fc files as a fatal error.
An empty fc file should be treated as "nothing to check"
so long as the -e option is passed.

We add this option, so we don't allow empty file_context
files to pass CTS checking.

Change-Id: Ibca6bd948a13389e10c605d613acc48c5504443e
Signed-off-by: William Roberts <william.c.roberts@intel.com>