kconfig: fix localmodconfig

When kconfig syntax moved to use $(FOO) for environment variables
localmodconfig was not updated.
Fix so it now works with the new syntax $(FOO)

Fixes: 104daea1 ("kconfig: reference environment variables directly and remove 'option env='")
Reported-by: Kevin Locke <kevin@kevinlocke.name>
Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>