localmodconfig: Add debug environment variable LOCALMODCONFIG_DEBUG

If the environment variable LOCALMODCONFIG_DEBUG is set, then debug output
will appear in the make localmodconfig. This will simplify debugging what
people get with their output, as I can just tell people to do:

  LOCALMODCONFIG_DEBUG=1 make localmodconfig 2>out.txt

and have them send me the out.txt. I'll be able to see why things are not
working as they think it should be.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>