Refactor TWI response system

- Avoid setting respondFlag in main, instead let it be managed directly
inside twiListen() and read inside twiRespond()

firmware/atmega/source/main.c

@@ -11,7 +11,6 @@
 
 volatile uint16_t lastMoistureVal = 0;
 volatile uint16_t *pLastMoistureVal = &lastMoistureVal;
-extern bool respondFlag;
 
 ISR(TWI_vect) {
     twiListen();
@@ -49,11 +48,7 @@ int main(void) {
         *pLastMoistureVal = moistureRead();
         // nested if: (result outside range (see config.h)) {waterPlant()}
 
-
-        if(respondFlag) {
         twiRespond();
-            respondFlag = false;
-        }
 
         enterSleep();
     }

firmware/atmega/source/twi.c

@@ -35,7 +35,7 @@ static void twiSendData(uint8_t data) {
 }
 
 void twiRespond(void) {
-    //FIXME: refactor this implementation
+    if(respondFlag) {
         switch(twiBuffer[0]) {
             case 0x01: // Send moisture
                 twiSendData(*pLastMoistureVal);
@@ -67,6 +67,7 @@ void twiRespond(void) {
                 twiSendNack();
                 break;
         }
+    }
 }
 
 void twiListen(void) {