Move check for disk-image-get-kernel

disk-image-get-kernel is only used if the vmlinuz and initrd are not
present in the directory with the image file. So there's no need to
check it's path which results in noise to the user with a big echo of
$PATH unless we intend to use it.

Change-Id: Ic972cb1ff5e9a1ccf5be07ed64af588059a3c6a5
Merge "Move check for disk-image-get-kernel"