Fixes '[: ==: unary operator expected'

When running clean_kernel.sh, the following error message was displayed:
./clean_kernel.sh: line 19: [: ==: unary operator expected

Even though the script still works, it's a bit irritating, so here's the fix for it.
For more info about why it's needed see:
http://stackoverflow.com/questions/408975/compare-integer-in-bash-unary-operator-expected/605138#605138