Due to the new MSD boot feature the RPi folks added for the pi4 and in an effort to streamline where our images can be burned to a sdcard or a MSD boot drive or kernel upgradesdepending on how you are booting I have changed in /boot/cmdline.txt how the root partition is defined. Instead of using the root partition's blk device it now will use the root partition's LABEL. Credit: @mitchejj
Changed from: root=/dev/mmcblk0p2 To: root=LABEL=ROOT_MNJRO
Our later images have had partition LABEL's BOOT_MNJRO & ROOT_MNJRO for a while and if you are running with these LABEL's all should be ok unless you have some other custom root= or custom cmdline.txt perimeter added.
On older images with different partition LABEL's then you can run into an issue booting after a kernel upgrade and the boot= needs to be like you had it (before booting in to the new kernel) or rename your partition LABEL's and be done with it. Depending if you or upgrading an existing kernel (Your cmdline.txt may not be over written but the new one will be written as cmdline.txt.pacsave) or if you are testing another kernel version (Then your cmdline.txt will be saved as cmdline.txt.pacsave).
As always if you had a custom config.txt then you need to check it before rebooting the new kernel also.
You still will have to flash the pi4 eeprom before trying MSD boot.
This change will start today as soon as I get all of the new kernels pushed to the unstable branch and eventually they will move down through the branches to the stable branch. The RPi tree's updated to new versions 2 days ago. Their new kernel versions along with their headers are:
linux-rpi4-4.19.126-1 linux-rpi4-next-5.4.44-1 linux-rpi4-mainline-5.6.16-1
Sorry about this change but it is in the best interest going forward.