update-rc.d.bbclass: Fix host/target test in postinst

When running the postinst script I get a shell warning:

sh: argument expected

and the service is never stopped. This patch fixes the warning
message and stops the service.

Patch v2: Hans Beckérus pointed out that the patch is not correct.
          This version uses the syntax proposed by Hans. I've tested
          that the postinst script works correctly when run on the target,
          both when the init script exists and when it doesn't exist.

Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>