[Review] libutee: provide an implementation of putchar()

Address Etienne's comments.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>

lib/libutee/trace_ext.c

@@ -102,7 +102,12 @@ int putchar(int c)
 
 	if (trace_get_level() >= TRACE_PRINTF_LEVEL)
 		trace_ext_puts(str);
-	return c;
+	/*
+	 * From the putchar() man page:
+	 * "fputc(), putc() and putchar() return the character written as an
+	 * unsigned char cast to an int or EOF on error."
+	 */
+	return (int)(unsigned char)c;
 }
 
 #else
@@ -117,9 +122,9 @@ int puts(const char *str __unused)
 	return 0;
 }
 
-int putchar(int c __unused)
+int putchar(int c)
 {
-	return c;
+	return (int)(unsigned char)c;
 }
 
 #endif