Fix iSCSI boot over IPv6

- scripts/boot-ibft.sh:
  if address is IPv6, put it into brackets and always use prefix length
  of 64 rather than (invalid) netmask read from sysfs
- scripts/boot-network.sh:
  allow client address field to be enclosed in brackets and contain colons
  (needed for IPv6)
- lib/mkinitrd/bin/ipconfig.sh:
  allow client address field to be enclosed in brackets and contain colons
  and prefix length to be passed in netmask field (needed for IPv6)

References: bnc#830621
Signed-off-by: Michal Marek <mmarek@suse.cz>