linux, efi, arm*, fdt: Break FDT extra allocation space out into a #define

A certain amount of dynamic space is required for the handover from
GRUB/Linux-EFI-stub. This entails things like initrd addresses,
address-cells entries and associated strings.

But move this into a proper centralised #define rather than live-code
it in the loader.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/fdt.h

@@ -22,6 +22,9 @@
 #include <grub/types.h>
 #include <grub/symbol.h>
 
+/* Space required when preparing the /chosen node after boot has been called. */
+#define GRUB_EFI_LINUX_FDT_EXTRA_SPACE 0x400
+
 #define FDT_MAGIC 0xD00DFEED
 
 typedef struct {