[POWERPC] boot/Makefile CONFIG_ variable fixes

This corrects the names of two CONFIG_ variables.

Note that the CONFIG_MPC86XADS fix uncovers another bug
(with mpc866_ads_defconfig) that will require fixing:

<--  snip  -->

...
arch/powerpc/boot/dtc -O dtb -o arch/powerpc/boot/mpc866ads.dtb -b 0  /home/bunk/linux/kernel-2.6/git/linux-2.6/arch/powerpc/boot/dts/mpc866ads.dts
DTC: dts->dtb  on file "/home/bunk/linux/kernel-2.6/git/linux-2.6/arch/powerpc/boot/dts/mpc866ads.dts"
  WRAP    arch/powerpc/boot/cuImage.mpc866ads
powerpc64-linux-ld: arch/powerpc/boot/cuboot-mpc866ads.o: No such file: No such file or directory
make[2]: *** [arch/powerpc/boot/cuImage.mpc866ads] Error 1

<--  snip  -->

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>