bootloader: Add an arguments_count field to stage2_args

Some boot platforms pass a non-NULL-terminated list of args
to the loader, so store the count here to avoid having to copy
the list itself.

headers/private/kernel/boot/stage2_args.h

@@ -13,6 +13,7 @@
 typedef struct stage2_args {
 	size_t heap_size;
 	const char **arguments;
+	int32 arguments_count;
 	struct platform_stage2_args	platform;
 } stage2_args ;