Well, the hang up on switch to amdgpudrmfb was due to the kernel boot parameter
'acpi=off' I had to use to go beyond the 'loading initramfs' step. Initial
hangup in the latter step was due to a UEFI bug of the laptop.

Fortunately, the manufacturer provides a BIOS update with the tag 'UEFI
security enhancement'. After flashing the new BIOS, the laptop boots with
kernel 4.10-rc2, with a few errors related to amdgpu, which I'll report in a
new post.

Sorry for the inconvenience.