Makefile: Unify condition for mpc85xx reset vector

Use 'CONFIG_MPC85XX_HAVE_RESET_VECTOR && CONFIG_OF_SEPARATE' pattern
instead of 'CONFIG_MPC85XX_HAVE_RESET_VECTOR && !CONFIG_OF_EMBED' also in
OBJCOPYFLAGS_u-boot-nodtb.bin as this pattern is used in rest of Makefile.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Behún <kabel@kernel.org>