meta-obmc-machines: Witherspoon kernel override

This was an oversight when moving all of the systems to the 4.7 kernel
tree.

Fixes openbmc/openbmc#662 and fixes openbmc/openbmc#648.

Change-Id: Idc39a1e8b8406f72f1c2c20a6a37af06eb2ad002
Signed-off-by: Joel Stanley <joel@jms.id.au>
3 files changed