boot-directdisk: Fix kernel location after STAGING_KERNEL_DIR change
This catches up with the STAGING_KERNEL_DIR location change
and uses the correct variable to future proof this issue.
[YOCTO #2783]
(From OE-Core rev: 28715eff6dff3415b1d7b0be8cbb465c417e307f)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>