Grub2 uses a combination of menuentries and submenus. We can select a kenel as default in a manner like this:
1) Edit /etc/default/grub
GRUB_DEFAULT="1>10"
2) grub-mkconfig -o /boot/grub/grub.cfg
In this case, the 10 th menuentry kernel in the 1st submenu is set as default.
In the default setting of /etc/grub.d/10_linux, one kernel is assigned at the 0th menuentry. The other kernels are assigned at the 1st submenu in the 1st, 2nd, 3rd, ..., menuentry. So, if you prefere to set the 11th kernel in grub.cfg, then GRUB_DEFAULT="1>10".
Probably, you can modify this default setting by editing /etc/grub.d/10_linux, but I prefer to delete old kernels.
