Merge branch 'feature/support_iperf' into 'master'

lwip/esp32: support iperf

1. Add _exit() definition in syscalls.c
2. Fix a bug in sockets.c on which iperf depends

I'm now investigating performance of esp32, need iperf to be merged in to master branch for convience

See merge request !136

components/esp32/syscalls.c

@@ -142,6 +142,10 @@ int _open_r(struct _reent *r, const char * path, int flags, int mode) {
     return 0;
 }
 
+void _exit(int __status) {
+    abort();
+}
+
 ssize_t _write_r(struct _reent *r, int fd, const void * data, size_t size) {
     const char *data_c = (const char *)data;
     if (fd == STDOUT_FILENO) {