kconfig: use specific tokens instead of T_ASSIGN for assignments

kconfig: use specific tokens instead of T_ASSIGN for assignments

Currently, the lexer returns T_ASSIGN for all of =, :=, and +=
associating yylval with the flavor.

I want to make the generated lexer as simple as possible. So, the
lexer should convert keywords to tokens without thinking about the
meaning.

   =  ->  T_EQUAL
  :=  ->  T_COLON_EQUAL
  +=  ->  T_PLUS_EQUAL

Unfortunately, Kconfig uses = instead of == for the equal operator.
So, the same token T_EQUAL is used for assignment and comparison.
The parser can still distinguish them from the context.

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