Target: Do not compile USART init code for unused USARTS

Signed-off-by: CTXz <ctx.xda@gmail.com>

target/main.c

@@ -77,6 +77,8 @@ typedef struct __attribute__((packed))
 
 volatile USART *usart;
 
+#if defined(USE_USART1)
+
 /* Intializes USART1
  * Returns the USART1 control register */
 USART *init_usart1()
@@ -98,6 +100,8 @@ USART *init_usart1()
 	return USART1;
 }
 
+#elif defined(USE_USART2)
+
 /* Intializes USART2
  * Returns the USART2 control register */
 USART *init_usart2()
@@ -119,6 +123,8 @@ USART *init_usart2()
 	return USART2;
 }
 
+#elif defined(USE_USART3)
+
 /* Intializes USART3
  * Returns the USART3 control register */
 USART *init_usart3()
@@ -138,6 +144,8 @@ USART *init_usart3()
 	return USART3;
 }
 
+#endif
+
 //// Printing
 
 const uint8_t txtMap[] = "0123456789ABCDEF";