sandbox: Set up global data before board_init_f()

At present sandbox defines CONFIG_SYS_GENERIC_GLOBAL_DATA, meaning that
the global_data pointer is set up in board_init_f().

If we set up and zero the global data before calling board_init_f() then we
don't need to define CONFIG_SYS_GENERIC_GLOBAL_DATA.

Make this change to simplify the init process.

Signed-off-by: Simon Glass <sjg@chromium.org>