kernel.bbclass: fix hardlink test

The commit [kernel.bbclass: test for hardlinks before installing] doesn't
work on all build machines without a space between the ! and the test for
hardlink equivalance. The build continues, while the test fails and no
copy is made into the sysroot. Eventually tasks like build_hdimg will fail
with the missing kernel with the root cause being less than obvious.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>