Comment 1 for bug 1326163

Reason not to be able choose between Android and Ubuntu at boot time is consequence of conscious decision to use unmodified Android/Ubuntu boot images. Also there is virtually zero support needed for future Android and Ubuntu releases, it just takes them unmodified.
While there is way to combine two boot images into one, they would still need to share one kernel build.
So to implement selection at boot time would require either change of bootloader to be able to choose from different boot images, or implement some sort of 'grub' boot image, which would hand over to target boot image. However this is beyond scope of this project.
For the moment, you can always hold down 'volume down' while booting phone and then select from bootloader option recovery mode, and phone boots to Ubuntu.

For switching default boot from Android to Ubuntu. Yes this is easy to implement feature and if time allows I will add it for next iteration...