Can't install Debian without dedicated /boot partition

[Albin <public@inquisitive.se>]

Hi!

I'm trying to install Debian Stretch with full disk encryption 
(LVM-on-LUKS). But when the installer is trying to install Grub in 
crashes with the following error message:

```
Unable to install GRUB in dummy
Executing 'grub-install dummy' failed.

This is a fatal error.
```

The disk is partitioned with an 512MiB (specified in bytes) EFI 
partition and the rest in an LUKS encrypted container. The container 
contains a LVM volume group with a swap partition and a root partition.

I'm able to install NixOS with GRUB without needing an separate /boot 
partition. So I believe it isn't a limitation in GRUB. Can't find any 
information in the documentation that says that a separate /boot 
partition is mandatory.

The ISO is written to the USB memory using Etcher with verification 
turned on. It was verified using SHA512 sums, which itself was verified 
with the GPG keys. I also ran the tool inside the Debian installer for 
verifying the installation media. The image I use is the netinstaller 
for Stretch (version 9.5).

Should I be able to install Debian Stretch without a separate /boot 
partition?

Thanks in advance!

Regards,
Albin