[POWERPC] Remove redundant reference to non-existent CONFIG_BOOTIMG

There is no BOOTIMG Kconfig variable, not to mention that there is no
include/linux/bootimg.h header file.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

diff --cc arch/ppc/platforms/ev64360.c
Simple merge

diff --cc arch/ppc/platforms/katana.c
Simple merge