kconfig: rename a variable in the lexer to a clearer name

In Kconfig, like Python, you can enclose a string by double-quotes or
single-quotes. So, both "foo" and 'foo' are allowed.

The variable, "str", is used to remember whether the string started with
a double-quote or a single-quote because open/closing quotation marks
must match.

The name "str" is too generic to understand the intent. Rename it to
"open_quote", which is easier to understand. The type should be 'char'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Boris Kolpackov <boris@codesynthesis.com>