part: let list put the list in an environment variable

Add an optional third argument to the "part list" command which puts a
space seperated list of valid partitions into the given environment
variable. This is useful for allowing boot scripts to iterate of all
partitions of a device.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Stephen Warren <swarren@nvidia.com>