kconfig: expand lefthand side of assignment statement

Make expands the lefthand side of assignment statements.  In fact,
Kbuild relies on it since kernel makefiles mostly look like this:

  obj-$(CONFIG_FOO) += foo.o

Do likewise in Kconfig.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/zconf.l

@@ -114,6 +114,13 @@ n	[A-Za-z0-9_-]
 		yylval.string = text;
 		return T_VARIABLE;
 	}
+	({n}|$)+	{
+		/* this token includes at least one '$' */
+		yylval.string = expand_token(yytext, yyleng);
+		if (strlen(yylval.string))
+			return T_VARIABLE;
+		free(yylval.string);
+	}
 	"="	{ BEGIN(ASSIGN_VAL); yylval.flavor = VAR_RECURSIVE; return T_ASSIGN; }
 	":="	{ BEGIN(ASSIGN_VAL); yylval.flavor = VAR_SIMPLE; return T_ASSIGN; }
 	"+="	{ BEGIN(ASSIGN_VAL); yylval.flavor = VAR_APPEND; return T_ASSIGN; }