fastboot: Support defining raw partitions without a partition table

Add support for defining raw fastboot partitions in eMMC by specifying
the offset and size in an environment variable. Optionally, the eMMC
hardware partition number may also be specified.

This makes it possible to e.g. update only part of the eMMC boot
partition, instead of having to write the entire partition.

Signed-off-by: Filip Brozovic <fbrozovic@gmail.com>