Cellular: Rewrite AT handler consume to tag

If sequence from buffer contains tag but symbol before tag is same as
first symbol of the tag, then the tag wasn't detected.

For example, "\r\n" tag was not found from "\r\r\nOK" sequence.

Author: Mirela Chirica

features/cellular/framework/AT/ATHandler.cpp

@@ -952,25 +952,31 @@ bool ATHandler::consume_char(char ch)
 bool ATHandler::consume_to_tag(const char *tag, bool consume_tag)
 {
     size_t match_pos = 0;
+    size_t tag_length = strlen(tag);
 
     while (true) {
         int c = get_char();
         if (c == -1) {
-            break;
-            // compares c against tag at current position and if this match fails
-            // compares c against tag[0] and also resets match_pos to 0
-        } else if (c == tag[match_pos] || ((match_pos = 1) && (c == tag[--match_pos]))) {
+            tr_debug("consume_to_tag not found");
+            return false;
+        }
+        if (c == tag[match_pos]) {
             match_pos++;
-            if (match_pos == strlen(tag)) {
-                if (!consume_tag) {
-                    _recv_pos -= strlen(tag);
-                }
-                return true;
+        } else if (match_pos != 0) {
+            match_pos = 0;
+            if (c == tag[match_pos]) {
+                match_pos++;
             }
         }
+        if (match_pos == tag_length) {
+            break;
+        }
     }
-    tr_debug("consume_to_tag not found");
-    return false;
+
+    if (!consume_tag) {
+        _recv_pos -= tag_length;
+    }
+    return true;
 }
 
 bool ATHandler::consume_to_stop_tag()