tools: efivar.py without arguments

When tools: efivar.py is called without arguments an error occurs:

Traceback (most recent call last):
  File "tools/efivar.py", line 380, in <module>
    main()
  File "tools/efivar.py", line 360, in main
    args.func(args)
AttributeError: 'Namespace' object has no attribute 'func'

Show the online help if the arguments do not specify a function.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>