powerpc/bootwrapper: Pad .dtb by default

Since most bootloaders or wrappers tend to update or add some information
to the .dtb they a handled they need some working space to do that in.

By default add 1K of padding via a default setting of DTS_FLAGS.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>