chimney,
Have a look at man clr-boot-manager in your terminal.
Briefly:
clr-boot-mangager list will list kernels, and show * beside the one that boots by default
clr-boot-mangager get-timeout should show how long it times-out before booting the default kernel
and the one that may solve your problem:
clr-boot-manager set-timeout which sets the timeout
So, if you set the time-out to zero, then there is no time-out and the default kernel will boot without having to press the Enter key
clr-boot-manager set-timeout 0