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>

hello_world/Makefile

@@ -1,7 +1,7 @@
 ARCH = $(shell uname -m)
 ifneq ("$(ARCH)", "ppc64")
 ifneq ("$(ARCH)", "ppc64le")
-        CROSS_COMPILE = powerpc64le-linux-
+        CROSS_COMPILE ?= powerpc64le-linux-
         endif
         endif
 

hello_world/console.c

@@ -1,8 +1,8 @@
-#include <unistd.h>
-#include <string.h>
 #include <stdint.h>
 #include <stdbool.h>
 
+#include "console.h"
+
 /*
  * Core UART functions to implement for a port
  */

hello_world/console.h

@@ -1,3 +1,5 @@
+#include <stddef.h>
+
 void potato_uart_init(void);
 int getchar(void);
 void putchar(unsigned char c);

hello_world/hello_world.c

@@ -1,5 +1,3 @@
-#include <unistd.h>
-#include <string.h>
 #include <stdint.h>
 #include <stdbool.h>