Create two partitions in the existing space at the beginning of the
image (8MB). These are intended to hold u-boot SPL and u-boot proper.
The partition types selected are compatible with SiFive boards, e.g. the
HiFive Unmatched. They can easily be overridden for a platform that uses
the same scheme but different partition types, e.g. the StarFive
It is not as simple to create the firmware partitions after the fact,
e.g. with partition indices 3 and 4. It is a shortcoming of current day
u-boot that the SPL loader looks for a specific partition index, rather
than the partition type, meaning that we will fail to boot if partition
2 doesn't contain u-boot.
Thus, our GENERICSD images become more generically usable with current
RISC-V hardware/firmware platforms.