stretch and grub on LVM

[Thomas Lange]

I can confirm this problem on jessie (did not tested stretch yet).
Before trying to fix it for stretch, I like to understand what should
happen, and why the GRUB_PC/10-setup script does not work in this
situation.

In my test BOOT_DEVICE was aslso set to /dev/vg1/root and
BOOT_PARTITION was not set at all. Then GROOT was set to something like
(lvmid/2Mf5k7ul-..........)

Does anyone knows which values should be the right ones?
Which device/partition should grub be written to?

-- 
regards Thomas