envkernel.sh: fix integer compare used for string
Prevent stricter shells from crashing there if $POSTMARKETOS_ENVKERNEL_ENBALED is an empty string. It was reported that it crashes in ksh93, and busybox sh also complains about it.
Prevent stricter shells from crashing there if $POSTMARKETOS_ENVKERNEL_ENBALED is an empty string. It was reported that it crashes in ksh93, and busybox sh also complains about it.