Don't store a KMessage in kernel_args for the boot volume, only the buffer address/size.

Pointers in kernel_args are going to be changed to unconditionally use 64-bit
storage (to make kernel_args compatible with both the x86 and x86_64 kernels).
KMessage stores a pointer to its buffer, however since KMessage is used
outside of the boot code it is undesirable to change it to use 64-bit storage
for the pointer as it may add additional overhead on 32-bit builds. Therefore,
only store the buffer address and size and then construct a KMessage from
those in the kernel.

headers/private/kernel/boot/kernel_args.h

@@ -18,8 +18,6 @@
 #include <platform_kernel_args.h>
 #include <arch_kernel_args.h>
 
-#include <util/KMessage.h>
-
 #define CURRENT_KERNEL_ARGS_VERSION	1
 #define MAX_KERNEL_ARGS_RANGE		20
 
@@ -59,7 +57,9 @@ typedef struct kernel_args {
 	uint32		num_cpus;
 	addr_range	cpu_kstack[MAX_BOOT_CPUS];
 
-	KMessage	boot_volume;
+	// boot volume KMessage data
+	void		*boot_volume;
+	int32		boot_volume_size;
 
 	struct driver_settings_file *driver_settings;