bsdinstall: fix ESP detection for auto ZFS layout

Pass the list of user selected disks from zfsboot to bootconfig so that
the latter doesn't rely on ESP autodetection that apparently fails for
some cases, e.g. memstick installation with nvme (boot) and sata drives.

While here, fix printing of debug messages in bootconfig.

Reviewed by: bcran, imp, tsoome
Differential Revision: https://reviews.freebsd.org/D21930