[POWERPC] Fix some missing build dependencies in arch/powerpc/boot

[POWERPC] Fix some missing build dependencies in arch/powerpc/boot

This patch fixes a couple of missing dependencies in
arch/powerpc/boot/Makefile.  First, it ensures that the zlib.h header
is linked in before attempting to build gunzip_util.o, as it is,
building gunzip_util.o usually works, but not always depending on make
order.

Second, it makes the final images which are built using a dts
dependent on that dts, so the image will be correctly rebuilt if the
dts changes.  This in turn requires fixing the definition of the dts
variable.  CONFIG_DEVICE_TREE from Kconfig will have quotes around it,
which don't matter when passing the variable to a shell, but which
need to be removed when incorporating it into a filename for make's
use.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>