scripts/config: replace hard-coded script name by a dynamic value

The script `config' prints its name in usage() function. It is currently
hard-coded to value `config'. However, the script may be reused under
a different name in contexts other than the Linux Kernel.

Replace the hard-coded value `config' by the name of the script at runtime.

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>
Acked-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>