Introduce geom_map, a GEOM provider designed for use by
embedded flash stores.

Some devices - notably those with uboot - don't have an
explicit partition table (eg like Redboot's FIS.)
geom_map thus provides an easy way to export the hard-coded
flash layout as geom providers for use by filesystems and
other tools.

It also includes a "search" function which allows for
dynamic creation of partition layouts where the device only
has a single hard-coded partition. For example, if
there is a "kernel+rootfs" partition, a single image can
be created which appends the rootfs after the kernel with
an appropriate search string. geom_map can be told to
search for said search string and create a partition
beginning after it.

Submitted by: Aleksandr Rybalko <ray@dlink.ua>