sys/shell: ensure character is flushed when echoing.

When using a serial terminal without local echo, the current line
would not get updated as the user typed because the shell module's
readline() was not flushing each character.

This commit fixes that behavior. For additional clarity, fflush is
turned into a macro (flush_if_needed) which expands to either a call
to fflush() or empty, according to the standard library used.

sys/shell/shell.c

@@ -44,6 +44,12 @@ static void _putchar(int c) {
 #endif
 #endif
 
+#ifdef MODULE_NEWLIB
+#define flush_if_needed()   (fflush(stdout))
+#else
+#define flush_if_needed()
+#endif
+
 static shell_command_handler_t find_handler(const shell_command_t *command_list, char *command)
 {
     const shell_command_t *command_lists[] = {
@@ -263,6 +269,7 @@ static int readline(char *buf, size_t size)
             *line_buf_ptr++ = c;
 #ifndef SHELL_NO_ECHO
             _putchar(c);
+            flush_if_needed();
 #endif
         }
     }
@@ -275,9 +282,7 @@ static inline void print_prompt(void)
     _putchar(' ');
 #endif
 
-#ifdef MODULE_NEWLIB
-    fflush(stdout);
-#endif
+    flush_if_needed();
 }
 
 void shell_run(const shell_command_t *shell_commands, char *line_buf, int len)