[PATCH v1] Support DHCPv6 and UEFI IPv6 PXE

[Michael Chang]

This patch set tries to make support of configuring IPv6 network interface
through the DHCPv6 protocol. A new command, net_bootp6, is therefore introduced
to serve the purpose. You can think it as DHCPv6/IPv6 version of the existing
net_bootp command.

In addition to that, the UEFI IPv6 PXE support can be easily done by sharing
the same routine with net_bootp6 to parse DHCPv6 Reply packet cached in the
firmware.

changes in v1:
- Added upper boundary check in find_dhcpv6_option
- Fix memory leak and freeing NULL pointer
- Improved error message logging to not get lost
- Use grub_cpu_to_be16_compile_time for endianess conversion when appropriate
- Removed grub_dhcpv6_dns_servers structure and use 16 bytes blocks
- Avoud magic numbers and use more descriptive sizeof when populating netbuff
- Move include/grub/efi/api.h to UEFI IPv6 PXE support patch
- Document the net_bootp6 command