sandbox: Add improved RAM simulation

Using mmap to allocate memory from the OS for RAM simulation we can use
u-boot own malloc implementation.

Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Matthias Weisser <weisserm@arcor.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

include/os.h

@@ -76,3 +76,11 @@ void os_exit(int exit_code);
  * Put tty into raw mode to mimic serial console better
  */
 void os_tty_raw(int fd);
+
+/**
+ * Acquires some memory from the underlying os.
+ *
+ * \param length	Number of bytes to be allocated
+ * \return Pointer to length bytes or NULL on error
+ */
+void *os_malloc(size_t length);