kconfig: refactor pattern matching in STRING state

Here, similar matching patters are duplicated in order to look ahead
the '\n' character. If the next character is '\n', the lexer returns
T_WORD_QUOTE because it must be prepared to return T_EOL at the next
match.

Use unput('\n') trick to reduce the code duplication.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>