Grub2 does not allow choice of "mainline kernel" vs. stock Ubuntu kernel.
"The mainline kernels have their own ABI namespace so they install side by side with the stock Ubuntu kernels (each kernel has a separate directory under /lib/modules/VERSION for example). This means that you can keep several mainline and Ubuntu stock kernels installed at the same time and select the one you need from the GRUB boot menu."
Here's from /boot:
Now in fact /boot/grub/grub.cfg does show entries for both kernels, but the Grub2 menu on boot does not show the stock Ubuntu kernel or allow choosing it.
How do I get Grub2 boot menu to display all the kernels that are in grub.cfg?