Use fall through comment to support gcc<7

Author: earlephilhower

cores/esp8266/libb64/cdecode.cpp

@@ -43,7 +43,7 @@ static int base64_decode_block_signed(const int8_t* code_in, const int length_in
           fragment = (int8_t)base64_decode_value_signed(*codechar++);
         } while (fragment < 0);
         *plainchar    = (fragment & 0x03f) << 2;
-         __attribute__ ((fallthrough));
+         // fall through
       case step_b:
         do {
           if (codechar == code_in+length_in){
@@ -55,7 +55,7 @@ static int base64_decode_block_signed(const int8_t* code_in, const int length_in
         } while (fragment < 0);
         *plainchar++ |= (fragment & 0x030) >> 4;
         *plainchar    = (fragment & 0x00f) << 4;
-         __attribute__ ((fallthrough));
+         // fall through
       case step_c:
         do {
           if (codechar == code_in+length_in){
@@ -67,7 +67,7 @@ static int base64_decode_block_signed(const int8_t* code_in, const int length_in
         } while (fragment < 0);
         *plainchar++ |= (fragment & 0x03c) >> 2;
         *plainchar    = (fragment & 0x003) << 6;
-         __attribute__ ((fallthrough));
+         // fall through
       case step_d:
         do {
           if (codechar == code_in+length_in){

cores/esp8266/libb64/cencode.cpp

@@ -50,7 +50,7 @@ int base64_encode_block(const char* plaintext_in, int length_in, char* code_out,
       result = (fragment & 0x0fc) >> 2;
       *codechar++ = base64_encode_value(result);
       result = (fragment & 0x003) << 4;
-       __attribute__ ((fallthrough));
+       // fall through
   case step_B:
       if (plainchar == plaintextend){
         state_in->result = result;
@@ -61,7 +61,7 @@ int base64_encode_block(const char* plaintext_in, int length_in, char* code_out,
       result |= (fragment & 0x0f0) >> 4;
       *codechar++ = base64_encode_value(result);
       result = (fragment & 0x00f) << 2;
-       __attribute__ ((fallthrough));
+       // fall through
   case step_C:
       if (plainchar == plaintextend){
         state_in->result = result;

libraries/ESP8266AVRISP/src/ESP8266AVRISP.cpp

@@ -109,8 +109,7 @@ AVRISPState_t ESP8266AVRISP::serve() {
             break;
         case AVRISP_STATE_PENDING: {
             _state = AVRISP_STATE_ACTIVE;
-        // fallthrough
-             __attribute__ ((fallthrough));
+            // fall through
         }
         case AVRISP_STATE_ACTIVE: {
             while (_client.available()) {