ibm: Create p10bmc build target

Replace rainier with p10bmc as build target. Keep the rainier.conf as a
symlink to p10bmc.conf to allow the transition. Remove the rainier.conf
and the machine override from p10bmc.conf in a later commit to complete
the replacement.

Tested: Built rainier and p10bmc and checked for some files/service
        units to exist on both.

Change-Id: Ic8c8c6ffbc3507fbd9c692da259e37ae695359a6
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>