Add stderr support and remove 1us timeout for timeouts of 0us (raspberrypi#858)

Author: kilograham

src/rp2_common/pico_stdio/stdio.c

@@ -29,6 +29,10 @@
 #include "pico/stdio_semihosting.h"
 #endif
 
+#define STDIO_HANDLE_STDIN  0
+#define STDIO_HANDLE_STDOUT 1
+#define STDIO_HANDLE_STDERR 2
+
 static stdio_driver_t *drivers;
 static stdio_driver_t *filter;
 
@@ -131,11 +135,13 @@ static int stdio_get_until(char *buf, int len, absolute_time_t until) {
                 }
             }
         }
+        if (time_reached(until)) {
+            return PICO_ERROR_TIMEOUT;
+        }
         // we sleep here in case the in_chars methods acquire mutexes or disable IRQs and
         // potentially starve out what they are waiting on (have seen this with USB)
         busy_wait_us(1);
-    } while (!time_reached(until));
-    return PICO_ERROR_TIMEOUT;
+    } while (true);
 }
 
 int WRAPPER_FUNC(putchar)(int c) {
@@ -165,14 +171,14 @@ int puts_raw(const char *s) {
 }
 
 int _read(int handle, char *buffer, int length) {
-    if (handle == 0) {
+    if (handle == STDIO_HANDLE_STDIN) {
         return stdio_get_until(buffer, length, at_the_end_of_time);
     }
     return -1;
 }
 
 int _write(int handle, char *buffer, int length) {
-    if (handle == 1) {
+    if (handle == STDIO_HANDLE_STDOUT || handle == STDIO_HANDLE_STDERR) {
         stdio_put_string(buffer, length, false, false);
         return length;
     }