arm: switch linux loader to linux_arm_kernel_header struct

Use kernel header struct and magic definition to align (and coexist) with
i386/arm64 ports.

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

include/grub/arm/linux.h

@@ -20,11 +20,20 @@
 #ifndef GRUB_ARM_LINUX_HEADER
 #define GRUB_ARM_LINUX_HEADER 1
 
-#define LINUX_ZIMAGE_OFFSET 0x24
-#define LINUX_ZIMAGE_MAGIC  0x016f2818
-
 #include "system.h"
 
+#define GRUB_LINUX_ARM_MAGIC_SIGNATURE 0x016f2818
+
+struct linux_arm_kernel_header {
+  grub_uint32_t code0;
+  grub_uint32_t reserved1[8];
+  grub_uint32_t magic;
+  grub_uint32_t start; /* _start */
+  grub_uint32_t end;   /* _edata */
+  grub_uint32_t reserved2[4];
+  grub_uint32_t hdr_offset;
+};
+
 #if defined GRUB_MACHINE_UBOOT
 # include <grub/uboot/uboot.h>
 # define LINUX_ADDRESS        (start_of_ram + 0x8000)