common, itest: pass u-boot env variables to itest.s

compare two U-Boot Environment variables with itest.s, example:

=> print tmp ver
tmp=U-Boot 2013.10-g75e
ver=U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
=> print check_ub_ver
check_ub_ver=if itest.s \${tmp} == \${ver}; then echo equal; else echo diff ;fi
=> run check_ub_ver
diff
=> setenv tmp U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
=> print tmp ver
tmp=U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
ver=U-Boot 2013.10-g75eb4bc (Jan 21 2014 - 10:35:39)MPC83XX
=> run check_ub_ver
equal

Signed-off-by: Heiko Schocher <hs@denx.de>

common/cmd_itest.c

@@ -71,6 +71,19 @@ static char * evalstr(char *s)
 	/* if the parameter starts with a * then assume a string pointer else its a literal */
 	if (s[0] == '*') {
 		return (char *)simple_strtoul(&s[1], NULL, 16);
+	} else if (s[0] == '$') {
+		int i = 2;
+
+		if (s[1] != '{')
+			return NULL;
+
+		while (s[i] != '}') {
+			if (s[i] == 0)
+				return NULL;
+			i++;
+		}
+		s[i] = 0;
+		return  getenv((const char *)&s[2]);
 	} else {
 		return s;
 	}