kconfig: test for /boot/config-uname after /proc/config.gz in localconfig

Many distros put their config in /boot/config-`uname -r`, add a check
for that right after /proc/config.gz

Reported-by: Alan Jenkins <sourcejedi.lkml@googlemail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>