powerpc: Cleanup BOOTFLAG_* references

Now that warm booting is not supported, there isn't a need for the
BOOTFLAG_COLD and BOOTFLAG_WARM defines, so remove them.

Note that this change makes the board info bd_bootflags field useless.
It will always be set to 0, but we leave it around so that we don't
break the board info structure that some OSes are expecting to be passed
from U-Boot.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>