sys: util: add a sign_extend function

Add a sign_extend function, same as the Linux sign_extend32 one. This is
useful for sign extending values smaller than 32 bits.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

include/zephyr/sys/util.h

@@ -562,6 +562,19 @@ static inline uint8_t bin2bcd(uint8_t bin)
  */
 uint8_t u8_to_dec(char *buf, uint8_t buflen, uint8_t value);
 
+/**
+ * @brief Sign extend an 8, 16 or 32 bit value using the index bit as sign bit.
+ *
+ * @param value The value to sign expand.
+ * @param index 0 based bit index to sign bit (0 to 31)
+ */
+static inline int32_t sign_extend(int32_t value, int index)
+{
+	uint8_t shift = 31 - index;
+
+	return (int32_t)(value << shift) >> shift;
+}
+
 /**
  * @brief Properly truncate a NULL-terminated UTF-8 string
  *