unix: Add option to use uPy readline, and enable by default.

This gets uPy readline working with unix port, with tab completion and
history.  GNU readline is still supported, configure using
MICROPY_USE_READLINE variable.

unix/main.c

@@ -278,8 +278,6 @@ STATIC void set_sys_argv(char *argv[], int argc, int start_arg) {
 #endif
 
 int main(int argc, char **argv) {
-    prompt_read_history();
-
     mp_stack_set_limit(40000 * (BYTES_PER_WORD / 4));
 
     pre_process_options(argc, argv);
@@ -445,7 +443,9 @@ int main(int argc, char **argv) {
     }
 
     if (ret == NOTHING_EXECUTED) {
+        prompt_read_history();
         ret = do_repl();
+        prompt_write_history();
     }
 
     #if MICROPY_PY_MICROPYTHON_MEM_INFO
@@ -463,7 +463,6 @@ int main(int argc, char **argv) {
 #endif
 
     //printf("total bytes = %d\n", m_get_total_bytes_allocated());
-    prompt_write_history();
     return ret & 0xff;
 }