Failed to update dtb if the partition is not mounted

Hi,

I found that the device tree can't be updated correctly if the partition is not mounted first, and the snapd will complain the following error message

"""
error: cannot perform the following tasks:
- Update assets from kernel "mediatek-genio-kernel" (unset) (cannot prepare update for volume structure #2 ("firmware") on volume pc: structure 2 on volume pc does not have a writable mountpoint in order to update the filesystem content)
"""

Please see the following gadget.yaml that I am using

"""
volumes:
  pc:
    schema: gpt
    # bootloader configuration is shipped and managed by snapd
    bootloader: grub
    structure:
      # In Mediatek genio chipset platforms, the 1st bootloader is the prebuilt fip.bin
      # fip.bin contains BL31 TF-A (Trusted-Firmware A), BL32 OP-TEE OS, and BL33 U-Boot
      - name: bootloaders
        offset: 524288
        size: 4M
        type: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      - name: bootloaders_b
        size: 4M
        type: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      - name: firmware
        filesystem: vfat
        type: 384E979B-EB76-435A-A3A6-1A071DBAD91D
        size: 32M
        content:
          - source: $kernel:dtbs/dtbs/mediatek/
            target: FIRMWARE/mediatek/genio-350-evk/
      - name: firmware_b
        filesystem: vfat
        type: 384E979B-EB76-435A-A3A6-1A071DBAD91D
        size: 32M
        content:
          - source: $kernel:dtbs/dtbs/mediatek/
            target: FIRMWARE/mediatek/genio-350-evk/
      - name: dramk
        size: 524288
        type: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      - name: misc
        size: 1M
        type: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      - name: capsule
        filesystem: vfat
        type: EF,C12A7328-F81F-11D2-BA4B-00A0C93EC93B
        size: 32M
      - name: bootassets
        size: 32M
        type: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      - name: ubuntu-seed
        role: system-seed
        filesystem: vfat
        # UEFI will boot the ESP partition by default first
        type: EF,C12A7328-F81F-11D2-BA4B-00A0C93EC93B
        size: 1200M
        update:
          edition: 1
        content:
          - source: grubaa64.efi
            target: EFI/boot/grubaa64.efi
          - source: shim.efi.signed
            target: EFI/boot/bootaa64.efi
      - name: ubuntu-boot
        role: system-boot
        filesystem: ext4
        type: 83,0FC63DAF-8483-4772-8E79-3D69D8477DE4
        # whats the appropriate size?
        size: 750M
        update:
          edition: 1
        content:
          - source: grubaa64.efi
            target: EFI/boot/grubaa64.efi
          - source: shim.efi.signed
            target: EFI/boot/bootaa64.efi
      - name: ubuntu-save
        role: system-save
        filesystem: ext4
        type: 83,0FC63DAF-8483-4772-8E79-3D69D8477DE4
        size: 32M
      - name: ubuntu-data
        role: system-data
        filesystem: ext4
        type: 83,0FC63DAF-8483-4772-8E79-3D69D8477DE4
        size: 1G

"""