kconfig: Make localmodconfig handle environment variables

The commit 838a2e55
 kbuild: migrate all arch to the kconfig mainmenu upgrade

Broke make localmodconfig. The reason was that it added a
environment variable to the kconfig source, which the
streamline_config.pl could not handle.

This patch changes streamline_config.pl to handle kconfig sources
using environment variables in their names.

Cc: Arnaud Lacombe <lacombar@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>