sandbox: Use malloc() and free() from os layer

At present sandbox calls malloc() from various places in the OS layer and
this results in calls to U-Boot's malloc() implementation. It is better to
use the on in the OS layer, since it does not mix allocations with the
main U-Boot code.

Fix this by replacing calls with malloc() to os_malloc(), etc.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>