rtai-config now doesn't complain on single RTAI kernel

Previously, the user was warned if no kernel selection option was given
but the script would default to running kernel.  This covers the
majority of RTAI users.

The new behavior is as follows.  If no kernel selection option was
given, then the number of available rtai-config-* files is counted.

- If this number is 1, the script defaults to the running kernel
  * If that rtai-config was indeed for the running kernel, all is ok
  * If that rtai-config was for a different kernel, the script prints an
    error saying that it refuses to proceed because this could be not
    what the user wants.  The reason is that normally if the user has a
    single RTAI kernel, s/he would built booting in that kernel.  If
    s/he is in a different kernel, it is likely that _that_ kernel was
    supposed to be an RTAI kernel as well, but perhaps there was a
    problem with the installation.  Either way, it doesn't hurt to make
    the user clarify the intention.
- If this number is different from 1, it's ambiguous which kernel the
  user is referring to.  The script gives an error and refuses to
  proceed.  This is to make sure the user doesn't accidentally run the
  wrong script by forgetting the -v or -r option, especially in a
  script!

See issue #16 for further discussion.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>