The newly-introduced STAGE1_KCONFIG variable serves
to keep those kernel configuration options that are
required to be present in the kernel to boot.

- introduced generic stage2 subprofile (non-standalone)
- ported installer and rescue over to stage2/{install2,rescue}
- initial stage2/live (needs more work for sure)
- use make-initrd-propagator
- updated and somewhat extended doc/

NB: mind #26133, #26134