build: image: add SOC device variable

This creates the device variable SOC and adds it to DEFAULT_DEVICE_VARS.

It is supposed to replace target-specific SOC variables like ATH_SOC or
MTK_SOC and thus unify variable names across targets.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

include/image.mk

@@ -420,6 +420,7 @@ define Device/Init
   DEVICE_DTS :=
   DEVICE_DTS_CONFIG :=
   DEVICE_DTS_DIR :=
+  SOC :=
 
   BOARD_NAME :=
   UIMAGE_NAME :=
@@ -437,7 +438,7 @@ DEFAULT_DEVICE_VARS := \
   DEVICE_NAME KERNEL KERNEL_INITRAMFS KERNEL_INITRAMFS_IMAGE KERNEL_SIZE \
   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 BOARD_NAME UIMAGE_NAME SUPPORTED_DEVICES \
+  DEVICE_DTS_CONFIG DEVICE_DTS_DIR SOC BOARD_NAME UIMAGE_NAME SUPPORTED_DEVICES \
   IMAGE_METADATA KERNEL_ENTRY KERNEL_LOADADDR UBOOT_PATH DEVICE_VENDOR \
   DEVICE_MODEL DEVICE_VARIANT \
   DEVICE_ALT0_VENDOR DEVICE_ALT0_MODEL DEVICE_ALT0_VARIANT \