rlc: Start to move things out of the tbf into the rlc

Add remarks of possible broken behavior inside the tbf
routines. Move the preparation (and init) into the new
rlc.cpp file.
diff --git a/src/tbf.h b/src/tbf.h
index bf5f32d..e95ed20 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -86,6 +86,8 @@
 #define GPRS_RLCMAC_FLAG_TO_MASK	0xf0 /* timeout bits */
 
 struct gprs_rlc_data {
+	uint8_t *prepare(size_t block_data_length);
+
 	/* block history */
 	uint8_t block[RLC_MAX_LEN];
 	/* block len  of history */