cmd: sysboot: null check filename

Currently if ${bootfile} is unset and sysboot is invoked with no
filename specified then U-Boot will crash will a null-pointer
dereference. Add the missing check and a matching error print.

Fixes: 993c912d ("cmd: sysboot: Create a sysboot command dedicated file")
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>