Current U-Boot 2020.07, just checked it in git.toradex.com, still disables RPMSG unless M4 is started from U-Boot. See fdt_status_disabled(blob, off) call in my May 5 answer. You need to fix it first and only then expect RPMSG working starting M4 from Linux using imx_m4fwloader or rproc. iMX7 rproc as I understand got broken after FSL or someone else added support for iMX8. Yes, I made it working, but code is too dirty hack to share.
