[oe] image level user manipulation

[George C. Huntington III]

are there any classes or layers that let you create users on an image
for the 2011.03-maintenance branch?

i want to create some custom users and home directories.  I think I can
do it via the ROOTFS_POSTPROCESS_COMMAND, but it feels like there should
be a better way.  is there a way to run the addgroup and adduser tools
natively but in such a way that they work their magic on the root
filesystem being created?