fixes #6268 - PEERDNS only for the BOOTIF interface

We need to take DNS from DHCP only for the interface that is given via the
BOOTIF kernel option.
bootif=$(cat /proc/cmdline | grep -io 'BOOTIF=\S*' | tail -n1 | tr A-F a-f |  grep -io -E  '([[:xdigit:]]{2}-){5}[[:xdigit:]]{2}$')