Merge pull request #15 from jk-ozlabs/master

Update buildroot submodule
Update to latest linux & buildroot repositories

This switches the palmetto defconfig to use the open-power linux-tree,
rather than the powerkvm tree. We need to update buildroot at the same
time, to drop the linux patches, and add the ipmitool package.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>