Kernel update problem

I was told to update my kernel every couple weeks or so using the rpi-update command,

However, I use berryboot. I think this is interfering with the ability to upgrade kernel, as this is the output from that command:

You appear to be using a custom kernel file.
Skipping installation of new kernel, as bundled dtb files may be incompatible with your kernel.
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** Performing self-update
*** Relaunching after update
You appear to be using a custom kernel file.
Skipping installation of new kernel, as bundled dtb files may be incompatible with your kernel.
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
#############################################################
WARNING: This update bumps to rpi-4.9.y linux tree
Be aware there could be compatibility issues with some drivers
Discussion here:
https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=167934
##############################################################
Would you like to proceed? (y/N)
*** Downloading specific firmware revision (this will take a few minutes)
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 168 0 168 0 0 75 0 --:–:-- 0:00:02 --:–:-- 75
100 53.6M 100 53.6M 0 0 1110k 0 0:00:49 0:00:49 --:–:-- 625k
*** Updating firmware
*** As requested, not updating kernel
*** As requested, not updating kernel modules
*** Updating VideoCore libraries
*** Using HardFP libraries
*** Updating SDK
*** Running ldconfig
*** Storing current firmware revision
*** Deleting downloaded files
*** Syncing changes to disk
*** If no errors appeared, your firmware was successfully updated to 0ead2bdf97eaf8312e08febb3a75addf59010ed0
*** A reboot is needed to activate the new firmware

Id ask the berryboot people but they dont see ot have a forum.

ANy idea how to keep my kernal updated?