core-utils: Handle BOARD_KERNEL_SEPARATED_DTBO false

When defining BOARD_KERNEL_SEPARATED_DTBO either true/false, in
both cases this block get executed. This block handles only true,
 "" case and not handling false case.
Changing the check condition to true to execute the block only if
BOARD_KERNEL_SEPARATED_DTBO set as true.

Change-Id: I5f981b5f042b52dce11ceb4adab5b1a66a674355
1 file changed