MFC r339465: rc.initdiskless: add support for auxiliary NVRAM.
Currently, rc.inidiskless assumes that local system configuration changes are kept in some mountable file system. For example, nanobsd uses dedicated partition mounted as /cfg for this. However, small embedded devices like MIPS routers may have no enough flash space to keep full-blown file system but have only one or couple small flash blocks to keep persistent local configuration overrides. This change extends rc.initdiskless and introduces ability to run auxiliary command /conf/T/M/extract that is supposed to extract configuration overrides from such local storage. For example, the command /conf/default/etc/extract may contain something like: cd "$1" && bsdcpio --quiet -idu < /dev/map/cfg bsdcpio command extracts compressed archive from the storage to /etc assuming the storage is exposed by the kernel as /dev/map/cfg to userland.
PR: 204215