kconfig/conf: reduce the scope of `defconfig_file'

This variable is not used outside of main() so there is not much reason keeping
it global. Ensure it is initialized as gcc has no way to know that normal
execution path expect only one option switch to be given on the command line
(except when we request help). As a result, we always initialize
`defconfig_file' before using it.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>