wic: Add gpt-hybrid partition layout

wic: Add gpt-hybrid partition layout

Add support for formatting a disk with a hybrid MBR & GPT partition
scheme. In this scheme, the primary partitioning method is GPT, but a
valid MBR header is also written than can point to a subset of the GPT
partitions on the disk (any partitions marked with the `--mbr` flag will
be included in this MBR). The primary purpose of this method is to allow
for SoCs that can only find a bootloader in an MBR partition to use GPT
once the bootloader is running. As an example, older versions of the
Raspberry Pi firmware can only parse MBR partitions to find a kernel (or
other bootloader like u-boot), but once those have booted GPT partitions
can be used.

In addition to the partitions annotated with the `--mbr`, a "protective"
GPT partition of type 0xEE is added, as the existence of such a
partition is the indication to tooling that this a hybrid MBR and that
the GPT partition table should be parsed instead.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>