kconfig: expand file names

This will allow to use the following construct in source files:

config FOO
    string
    default "foo"

source "$FOO/Kconfig"

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