helpers/envkernel.sh: Try harder to not generate Git version suffix

The Git version suffixes usually generated automatically by the Linux build system break packaging in Alpine because the flavor (e.g. "-postmarketos-qcom-msm8916") will no longer match the module and zImage path. So far setting LOCALVERSION= and disabling CONFIG_LOCALVERSION_AUTO (see !1844 (merged)) was sufficient to avoid the Git version suffixes but this is no longer enough in Linux 5.14.

Instead, add a better fix by creating an empty .scmversion file that will be used by the kernel instead of checking the Git status. The advantage is that this works on Linux 5.14 and it should even work when CONFIG_LOCALVERSION_AUTO is set.