sandbox: Deal with conflicting getenv() for SDL

Unfortunately SDL requires getenv() to operate, since it wants to figure out
the display type. U-Boot has its own getenv() and they conflict. As a
work-around use #define to resolve the conflict.

A better but more complex solution might be to rename some U-Boot symbols
at link time. SDL audio is not functional at present, likely due to a related
issue.

Note: Vic Yank wrote a script for this, filed in crbug.com/271125.

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