local manifest
<?xml version="1.0" encoding="UTF-8"?>
<manifest>
        <project path="device/xiaomi/Mi439"
        name="LineageOS/android_device_xiaomi_Mi439"
        revision="lineage-21" />

        <project path="kernel/xiaomi/msm8937"
        name="LineageOS/android_kernel_xiaomi_msm8937"
        revision="lineage-21" />

        <project path="device/xiaomi/mithorium-common"
        name="LineageOS/android_device_xiaomi_mithorium-common"
        revision="lineage-21" />

        <project path="vendor/xiaomi/Mi439"
        name="TheMuppets/proprietary_vendor_xiaomi_Mi439"
        revision="lineage-21" />

        <project path="vendor/xiaomi/mithorium-common"
        name="proprietary_vendor_xiaomi_mithorium-common"
        revision="lineage-21" />
</manifest>

============================================

HABUILD_SDK [Mi439] renard@sfosBUILD:~/hadk$ breakfast $DEVICE
16:39:15 Build sandboxing disabled due to nsjail error.
In file included from build/make/core/config.mk:380:
In file included from build/make/core/envsetup.mk:369:
In file included from build/make/target/product/product_launched_with_p.mk:2:
device/xiaomi/Mi439/device.mk:107: error:  vendor/xiaomi/Mi439/Mi439-vendor.mk does not exist..
16:39:15 dumpvars failed with: exit status 1
Device Mi439 not found. Attempting to retrieve device repository from LineageOS Github (http://github.com/LineageOS).
Found repository: android_device_xiaomi_Mi439
Default revision: lineage-21.0
Checking branch info
Using fallback branch: lineage-21
Checking if device/xiaomi/Mi439 is fetched from android_device_xiaomi_Mi439
LineageOS/android_device_xiaomi_Mi439 already fetched to device/xiaomi/Mi439
Syncing repository to retrieve project.
Syncing: 100% (1/1), done in 0.516s
repo sync has finished successfully.
Repository synced!
Looking for dependencies in device/xiaomi/Mi439
Looking for dependencies in device/xiaomi/mithorium-common
Looking for dependencies in kernel/xiaomi/msm8937
kernel/xiaomi/msm8937 has no additional dependencies.
Done
16:39:18 Build sandboxing disabled due to nsjail error.
In file included from build/make/core/config.mk:380:
In file included from build/make/core/envsetup.mk:369:
In file included from build/make/target/product/product_launched_with_p.mk:2:
device/xiaomi/Mi439/device.mk:107: error:  vendor/xiaomi/Mi439/Mi439-vendor.mk does not exist..
16:39:18 dumpvars failed with: exit status 1
16:39:19 Build sandboxing disabled due to nsjail error.
In file included from build/make/core/config.mk:380:
In file included from build/make/core/envsetup.mk:369:
In file included from build/make/target/product/product_launched_with_p.mk:2:
device/xiaomi/Mi439/device.mk:107: error:  vendor/xiaomi/Mi439/Mi439-vendor.mk does not exist..
16:39:19 dumpvars failed with: exit status 1

** Don't have a product spec for: 'lineage_Mi439'
** Do you have the right repo manifest?