kconfig: test if a .config already exists

If one were to run localmodconfig or localyesconfig without having
a .config already in the file, then the end of the process would give
a warning when it tries to move the old .config to .config.old.

This patch adds a test to check if .config exists and avoid the moves
if it does not.

[ Impact: remove warning after make localmodconfig ]

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