gsm0808: Add utils for Encryption Information

The planned support for true A over IP requires the encoding of
the an Encryption Information element (see also BSS_MAP_MSG_CIPHER_MODE_CMD).

This commt adds encoding/decoding functionality and tests for
the element mentioned above, however, it is not yet actively used.

Change-Id: I8262050a9d9fd3f17462cfbb046c6e034dccc6fb
diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 48e737d..5bd27c7 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -55,3 +55,11 @@
 /* Decode Channel Type element */
 int gsm0808_dec_channel_type(struct gsm0808_channel_type *ct,
 			     const uint8_t *elem, uint8_t len);
+
+/* Encode Encryption Information element */
+uint8_t gsm0808_enc_encrypt_info(struct msgb *msg,
+				 const struct gsm0808_encrypt_info *ei);
+
+/* Decode Encryption Information element */
+int gsm0808_dec_encrypt_info(struct gsm0808_encrypt_info *ei,
+			     const uint8_t *elem, uint8_t len);