Moving /boot to another partition

[Martin]

Hi Grub-Team,
I have ubuntu 16.04 and my /boot directory has become too small (only 150MB) so 
I want to move it. For this I need some help.
I asked in ubuntuforums.org and askubuntu.com, but nobody could help.

/boot is on a separate ext4 partition with 150MB.
"/" is on a btrfs partition with 40GB.

My thoughts how to solve this:

    -create /boot2 on btrfs
    -copy all files from /boot to /boot2
    -unmount /boot
    -rename /boot2 to /boot
    -run grub-install /dev/sdb5   # ?

I'm not sure about the device for grub-install.
Is it the device where the grub partition is (sdb1) or is it the device where 
/boot is (sdb5)?


Here ist the output of parted -l:

    Model: ATA Samsung SSD 750 (scsi)
    Disk /dev/sdb: 120GB
    Sector size (logical/physical): 512B/512B
    Partition Table: gpt
    Disk Flags:

    Number  Start   End     Size    File system     Name      Flags
     1      1049kB  316MB   315MB                   bios      bios_grub
     2      316MB   473MB   157MB   ext4            boot
     3      473MB   8862MB  8389MB  linux-swap(v1)  swap
     5      8862MB  50,6GB  41,8GB  btrfs           root
     4      50,6GB  120GB   69,2GB  btrfs           home
     6      120GB   120GB   210MB   fat32           exchange  msftdata



This is what dpkg tells about grub2:

    ii  grub2-common                  2.02~beta2-36ubuntu amd64               
GRand Unified Bootloader (common files for version 2)

I do not use UEFI boot.

Thank you very much for helping,
Martin