Allow Print::println() to work with PROGMEM strings (#6450)

Adjust the ::write implementation in Print and its overridden copy in
UART to allow it to silentely accept PROGMEM strings (since there is no
write_P macro).

Fixes #6383

cores/esp8266/Print.cpp

@@ -45,7 +45,7 @@ size_t Print::write(const uint8_t *buffer, size_t size) {
 
     size_t n = 0;
     while (size--) {
-        size_t ret = write(*buffer++);
+        size_t ret = write(pgm_read_byte(buffer++));
         if (ret == 0) {
             // Write of last byte didn't complete, abort additional processing
             break;

cores/esp8266/Print.h

@@ -56,7 +56,7 @@ class Print {
         size_t write(const char *str) {
             if(str == NULL)
                 return 0;
-            return write((const uint8_t *) str, strlen(str));
+            return write((const uint8_t *) str, strlen_P(str));
         }
         virtual size_t write(const uint8_t *buffer, size_t size);
         size_t write(const char *buffer, size_t size) {

cores/esp8266/uart.cpp

@@ -495,7 +495,7 @@ uart_write(uart_t* uart, const char* buf, size_t size)
     size_t ret = size;
     const int uart_nr = uart->uart_nr;
     while (size--)
-        uart_do_write_char(uart_nr, *buf++);
+        uart_do_write_char(uart_nr, pgm_read_byte(buf++));
 
     return ret;
 }