sandbox: make RAM size configurable

sandbox: make RAM size configurable

Up to now the RAM size of the sandbox is hard coded as 128 MiB. This does
not allow testing the correct handling of addresses outside the 32bit
range. 128 MiB is also rather small when tracing functions where the trace
is written to RAM.

Provide configuration variable CONFIG_SANDBOX_RAM_SIZE_MB to set the RAM
size in MiB. It defaults to 128 MiB with a minimum of 64 MiB.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>