hello_world: Adjust header inclusions and Makefile

hello_world: Adjust header inclusions and Makefile

Currently hello_world fails to build with distro cross compiler
packages such as Debian gcc-powerpc64-linux-gnu, because it doesn't
provide string.h or unistd.h.  In fact we don't need them, we just
need stddef.h.  This adds #include <stddef.h> to console.h to get
size_t defined.  We also add #include "console.h" to console.c.

The hello_world Makefile currently hard-codes CROSS_COMPILE on
non-PPC machines.  This means that a command like:

	$ CROSS_COMPILE=powerpc64le-linux-gnu- make

doesn't do what you expect; it just tries to use powerpc64le-linux-gcc
regardless.  Adding a '?' makes it do what one expects.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>