@mautz
I am about to generate a new image with some changes. Can you take a look my question here, please? Thanks.

Edit: The mer-kernel-check says to unset CONFIG_BT_MSM_SLEEP. In my kernel that was y. Now I set it to n. Maybe this is why the bluetooth doesn't work?

Edit2: I also set CONFIG_TMPFS_XATTR as strongly recommended.

Edit3: and all of these:
CONFIG_CGROUPS
CONFIG_CGROUP_FREEZER
CONFIG_CGROUP_DEVICE
CONFIG_CGROUP_CPUACCT
CONFIG_CGROUP_MEM_RES_CTLR
CONFIG_CGROUP_MEM_RES_CTLR_SWAP
CONFIG_CGROUP_MEM_RES_CTLR_KMEM

Edit4: I think the unset CONFIG_BT_HCISMD is the reason why the bluetooth doesn't work. I go through the hadk-faq to try make it work.