classes/kernel-balena: Avoid re-building kernel modules when not signed

[acostach]

Setting the nostamp flag to any value, even if it is an empty string, causes the kernel modules to be rebuilt, which is contrary to the documentation: https://lists.openembedded.org/g/bitbake-devel/message/12359

Let's avoid setting it altogether on kernels which are not signed, to speed up builds.

Change-type: patch

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