kconfig check: allow skip with !pmb:kconfigcheck

Ignore APKBUILDs that have "!pmb:kconfigcheck" in their options by default in "pmbootstrap kconfig check", but print a note that they have been skipped. Check all kernels with "pmbootstrap kconfig check --all".

This is necessary, because the Librem 5 devboard kernel's config does not have CONFIG_DM_CRYPT enabled in their config, and we check for that. As the device is still under heavy development, we will make our lives easier by just using the upstream kernel config without any changes and ignoring it in our check by default.

(linux-purism-librem5dev still needs to be adjusted in pmaports.git.)

Related: pmaports#189 (closed)

Test together with: https://gitlab.com/postmarketOS/pmaports/merge_requests/215