ramips: use ARTIFACTS for initramfs-factory of I-O DATA WN-AX1167GR

ramips: use ARTIFACTS for initramfs-factory of I-O DATA WN-AX1167GR

Use ARTIFACTS to generate initramfs-based factory image of I-O DATA
WN-AX1167GR instead of redundant recipe which generate on
KERNEL_INITRAMFS.

Note:

WN-AX1167GR has 2x OS images on stock firmware.

stock log:

flash manufacture id: c2, device id 20 18
MX25L12805D(c2 2018c220) (16384 Kbytes)
mtd .name = raspi, .size = 0x01000000 (16M) .erasesize = 0x00010000 (64K) .numeraseregions = 0
Creating 10 MTD partitions on "raspi":
0x000000000000-0x000001000000 : "ALL"
0x000000000000-0x000000030000 : "Bootloader"
0x000000030000-0x000000040000 : "Config "
0x000000040000-0x000000050000 : "Factory"
0x000000050000-0x000000060000 : "iNIC_rf"
0x000000060000-0x0000007e0000 : "Kernel"
0x000000800000-0x000000f80000 : "app"
0x000000f90000-0x000000fa0000 : "Key"
0x000000fa0000-0x000000fb0000 : "backup"
0x000000fb0000-0x000001000000 : "storage"

1st image is "Kernel" and 2nd is "app" when booted from 1st image.
In OpenWrt, those 2x partitions are combined to "firmware" with
undefined (empty) areas (0x7e0000-0x7fffff, 0xf80000-0xf8ffff).

The size of an OS image partition is 0x780000 (7680 KiB = 7.5 MiB), so
check-size for initramfs-factory image needs to be called with the size.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>