ringbuffer: Don't print/TRAC from ringbuffer

In commit eac1bec428 we start to use the
ringbuffer inside the console printing code.  As a result, we must not
use TRACE_*() or printf() from within ringbuffer.c code to avoid
infinite recursion.

Instead, let rbuf_write() return a negative return value in case the
ring buffer overflows.  This way, the callers (outside the
console/stdout code) can print an error message themselves.

Change-Id: Ib009f013be119dbad22fa2b7d60ec8dee59baee5

firmware/libcommon/source/ringbuffer.c

@@ -2,6 +2,10 @@
 #include "trace.h"
 #include "utils.h"
 
+/* WARNINGI: Since console output is internally using this ringbuffer to implement
+ * buffered writes, we cannot use any TRACE_*() or printf() style functions here,
+ * as it would create infinite recursion! */
+
 void rbuf_reset(volatile ringbuf * rb)
 {
 	unsigned long state;
@@ -52,7 +56,7 @@ bool rbuf_is_full(volatile ringbuf * rb)
 	return rc;
 }
 
-void rbuf_write(volatile ringbuf * rb, uint8_t item)
+int rbuf_write(volatile ringbuf * rb, uint8_t item)
 {
 	unsigned long state;
 
@@ -61,9 +65,10 @@ void rbuf_write(volatile ringbuf * rb, uint8_t item)
 		rb->buf[rb->iwr] = item;
 		rb->iwr = (rb->iwr + 1) % RING_BUFLEN;
 		local_irq_restore(state);
+		return 0;
 	} else {
 		local_irq_restore(state);
-		TRACE_ERROR("Ringbuffer full, losing bytes!");
+		return -1;
 	}
 }