sandbox: Use memmove() to move overlapping regions

sandbox: Use memmove() to move overlapping regions

The use of strcpy() to remove characters at the start of a string is safe
in U-Boot, since we know the implementation. But in os.c we are using the
C library's strcpy() function, where this behaviour is not permitted.

Update the code to use memmove() instead.

Reported-by: Coverity (CID: 173279)
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexander Graf <agraf@suse.de>