build: add UIMAGE_MAGIC to device variables

Allow a device recipe to specify a custom UIMAGE_MAGIC value, as used by
OpenWrt's -M flag for mkimage. This allows to automatically customize
the magic bytes in all calls to Build/uImage for this device, similar to
the behaviour of UIMAGE_NAME. Since the -M argument is inserted before
the user arguments, it can be overriden.

The following example would use 0x87654321 for the KERNEL image, but
0x12345678 for the KERNEL_INITRAMFS image:

  define Device/MyDevice
    UIMAGE_MAGIC := 0x87654321
    KERNEL := ... | uImage lzma
    KERNEL_INITRAMFS := ... | uImage lzma -M 0x12345678
    ...
  endef

Fixes: df8e6be59a ("rtl838x: add new architecture")
[UIMAGE_MAGIC was not declared as a device variable]

Signed-off-by: Sander Vanheule <sander@svanheule.net>
[rebase, improve formatting of "Fixes"]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

include/image-commands.mk

@@ -402,6 +402,7 @@ define Build/uImage
 		-a $(KERNEL_LOADADDR) \
 		-e $(if $(KERNEL_ENTRY),$(KERNEL_ENTRY),$(KERNEL_LOADADDR)) \
 		-n '$(if $(UIMAGE_NAME),$(UIMAGE_NAME),$(call toupper,$(LINUX_KARCH)) $(VERSION_DIST) Linux-$(LINUX_VERSION))' \
+		$(if $(UIMAGE_MAGIC),-M $(UIMAGE_MAGIC)) \
 		$(wordlist 2,$(words $(1)),$(1)) \
 		-d $@ $@.new
 	mv $@.new $@

include/image.mk

@@ -400,6 +400,7 @@ define Device/Init
   SOC :=
 
   BOARD_NAME :=
+  UIMAGE_MAGIC :=
   UIMAGE_NAME :=
   DEVICE_COMPAT_VERSION := 1.0
   DEVICE_COMPAT_MESSAGE :=
@@ -419,7 +420,8 @@ DEFAULT_DEVICE_VARS := \
   CMDLINE UBOOTENV_IN_UBI KERNEL_IN_UBI BLOCKSIZE PAGESIZE SUBPAGESIZE \
   VID_HDR_OFFSET UBINIZE_OPTS UBINIZE_PARTS MKUBIFS_OPTS DEVICE_DTS \
   DEVICE_DTS_CONFIG DEVICE_DTS_DIR DEVICE_FDT_NUM SOC BOARD_NAME \
-  UIMAGE_NAME SUPPORTED_DEVICES IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR \
+  UIMAGE_MAGIC UIMAGE_NAME \
+  SUPPORTED_DEVICES IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR \
   UBOOT_PATH IMAGE_SIZE \
   DEVICE_COMPAT_VERSION DEVICE_COMPAT_MESSAGE \
   DEVICE_VENDOR DEVICE_MODEL DEVICE_VARIANT \