Hi Tad,
You can add a new .mk file to add the new subtarget.
Take the imx8 for example: target/linux/imx/image/imx8.mk.
But I don't think you need to add a new subtarget.
You really need to add a new device or called profile for your customized board.
If this is the case, you can add a new device item in
target/linux/imx/image/imx8.mk.
Take imx8mplus for example:
17 define Device/imx8mplus
18 DEVICE_VENDOR := NXP
19 DEVICE_MODEL := IMX8MPLUS
20 DEVICE_VARIANT := SD Card Boot
21 PLAT := iMX8MP
22 SOC_TYPE := iMX8M
23 DEVICE_TYPE := flash_evk
24 ENV_NAME:=imx8mp-sdboot
25 DEVICE_PACKAGES += \
26 atf-imx8mp \
27 firmware-imx \
28 imx-mkimage \
29 u-boot-imx8mp
30 DEVICE_DTS := freescale/imx8mp-evk
31 IMAGE/sdcard.img := \
32 imx-compile-dtb $$(DEVICE_DTS) | \
33 imx-create-flash $$(PLAT) $$(DEVICE_TYPE) | \
34 imx-clean | \
35 imx-append-sdhead $(1) | pad-to 32K | \
36 imx-append-boot $$(SOC_TYPE) | pad-to 4M | \
37 imx-append $$(ENV_NAME)-uboot-env.bin | pad-to $(IMX_SD_KERNELPART_OFFSET)M | \
38 imx-append-kernel $$(DEVICE_DTS) | pad-to $(IMX_SD_ROOTFSPART_OFFSET)M | \
39 append-rootfs | pad-to $(IMX_SD_IMAGE_SIZE)M
40 endef
41 TARGET_DEVICES += imx8mplus
