FIP tool: Fix error message for missing FIP filename

Previously to this path, the FIP tool used to print the following,
misleading error message when called without any argument or with
'--help' option:

  ERROR: Too many arguments

This patch fixes this behavior by printing the following error
message instead:

  ERROR: Missing FIP filename

If called with '--help', no error message is printed and only the
help message is displayed.

Change-Id: Ib281b056f5cd3bc2f66d98be0b0cb2a0aed7c6a8