sandbox: remove ram buffer file when U-Boot is loaded by SPL

sandbox: remove ram buffer file when U-Boot is loaded by SPL

Update management of "--rm_memory" sandbox's option and force
this option when U-Boot is loaded by SPL in os_spl_to_uboot()
and remove the ram file after reading in main() as described
in option help message: "Remove memory file after reading".

This patch avoids that the file "/tmp/u-boot.mem.XXXXXX" [created in
os_jump_to_file() when U-Boot is loaded by SPL] is never deleted
because state_uninit() is not called after U-Boot execution
(CtrlC or with running pytest for example).

This issue is reproduced by
> build-sandbox_spl/spl/u-boot-spl
  and CtrlC in U-Bot console

> make qcheck

One temp file is created after each SPL and U-Boot execution
(7 tims in qcheck after test_handoff.py, test_ofplatdata.py,
 test_spl.py execution).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>