powerpc/bootwrapper: Allow user to specify additional default targets

It is inconvenient to add additional default targets to the bootwrapper
Makefile for each new board supported which just needs a different dts
file.  This change allows the defconfig to specify additional build
targets.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>