Fix PROGMEM addressing in debug.h

I hate when the compiler doesn't check something what it normally does:
`pgm_read_byte` is more than happy with a parameter (*str), which reads
an address at a location where *str points to - which is obviously not the intent.

src/debug.cpp

@@ -22,12 +22,12 @@ const char hal[] PROGMEM = "hal:";
 #endif
 
 void dbg_usart(const char *layer_P, const char *s) {
-    hu::usart1.puts_P(layer_P);
+    hu::usart1.WriteS_P(layer_P);
     hu::usart1.puts(s);
 }
 
 void dbg_usart_P(const char *layer_P, const char *s_P) {
-    hu::usart1.puts_P(layer_P);
+    hu::usart1.WriteS_P(layer_P);
     hu::usart1.puts_P(s_P);
 }
 

src/hal/avr/usart.cpp

@@ -65,18 +65,26 @@ void USART::Flush() {
     // the hardware finished tranmission (TXC is set).
 }
 
-void USART::puts(const char *str) {
-    while (*str) {
+void USART::WriteS(const char *str) {
+    while (*str != 0) {
         Write(*str++);
     }
+}
+
+void USART::WriteS_P(const char *str_P) {
+    char c = 0;
+    while ((c = pgm_read_byte(str_P++)) != 0) {
+        Write(c);
+    }
+}
+
+void USART::puts(const char *str) {
+    WriteS(str);
     Write('\n');
 }
 
-void USART::puts_P(const char *str) {
-    char c;
-    while ((c = pgm_read_byte(*str++)) != 0) {
-        Write(c);
-    }
+void USART::puts_P(const char *str_P) {
+    WriteS_P(str_P);
     Write('\n');
 }
 

src/hal/usart.h

@@ -45,12 +45,19 @@ public:
 
     /// @param c character to be pushed into the TX buffer (to be sent)
     void Write(uint8_t c);
+    /// @param str pointer to a string in RAM to be pushed byte-by-byte into the TX buffer (to be sent)
+    /// No NL character is appended
+    void WriteS(const char *str);
+    /// @param str pointer to a string in PROGMEM to be pushed byte-by-byte into the TX buffer (to be sent)
+    /// No NL character is appended
+    void WriteS_P(const char *str_P);
+
     /// @param str c string to be sent. NL is appended
     /// Works on RAM strings
     void puts(const char *str);
     /// @param str c string to be sent. NL is appended
     /// Works on PROGMEM strings
-    void puts_P(const char *str);
+    void puts_P(const char *str_P);
     /// @returns true if there is at least one byte free in the TX buffer (i.e. some space to add a character to be sent)
     bool CanWrite() const {
         return !tx_buf.full();