I am getting this same error using Elementary OS with kernel 4.15rc7 and 4.15rc8 but not with 4.4 or 4.14.
Model: HP Envy x360 Convertible 15-bq100
Processor: AMD Ryzen 2500U
BIOS Mode: UEFI
Edit: I am getting the error with 4.14 but the laptop does complete booting.
Because I had a working version I did
journalctl -b1to view previous boot logs. It showed I didn't have Raven firmware. I got that from https://people.freedesktop.org/~agd5f/radeon_ucode/raven/. I am not sure if that is safe but I put it in /libs/firmware/amdgpu/ and then
update-initramfs -uto update the boot image to include the firmware. This is probably different since you don't have Raven Ridge.
I couldn't view previous boot logs so I followed dma_k's instructions in the first answer of this post How to find previous boot log after Ubuntu 16.04 restarts? - Ask Ubuntu
I am also experiencing a similar issue with the ryzen HP envy x360.
I am running arch linux and I have kernel 4.15. My computer boots up and runs okay mostly, The touchscreen (ELAN 0732 driver in windows) is not recognized by wacom drivers, I cannot find it in lspci or libwacom-list-local-devices. I was wondering if it is related to GPIO error messages I get while booting up.
I am aware AMD is working on ryzen support in coming kernels, was just wondering what I could expect from future releases and if my touchscreen issues are related to chipset support.