configure: Avoid '==' bashism

The '==' operator to test is a bashism; the standard way to copmare
strings is '='. This causes dash to complain:

../../configure: 681: test: linux: unexpected operator

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20220720152631.450903-6-peter.maydell@linaro.org