ibm: palmetto: Set QB_MACHINE

Palmetto has a Qemu model called palmetto-bmc.

Tested: bitbake core-image-minimal && runqemu slirp nographic

Change-Id: Ieea6bd5805c36a6d18e22e5aa6e18290b8026283
Signed-off-by: Joel Stanley <joel@jms.id.au>