ASCII: Add function for converting two hex chars to value.

utils/ascii.h

@@ -187,6 +187,19 @@ static inline int ascii_hex_to_value(char c)
 	return -256;
 }
 
+/**
+ * Converts two hexadecimal characters to a single number
+ *
+ * \param[in] c1  most significant hex digit.
+ * \param[in] c2  least significant hex digit.
+ * \return the total value of the two digit hex number (0-255),
+ *         or -ve if input not hex.
+ */
+static inline int ascii_hex_to_value_2_chars(char c1, char c2)
+{
+	return 16 * ascii_hex_to_value(c1) + ascii_hex_to_value(c2);
+}
+
 /**
  * Convert an upper case character to lower case.
  *