meta-ibm: Move Tacoma and Rainier to eMMC u-boot config

The AST2600 systems booted from NOR during bringup, but now that we have
eMMC support in u-boot they can run from eMMC.

(From meta-ibm rev: 46419c51c3bd3d43873ecbab7fbbb6f1d6c54de4)

Signed-off-by: Joel Stanley <joel@jms.id.au>
Change-Id: Idd2389ab62e1b7cc32639f5547f1f1098631654f
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>