To make grub2 remember your last choice you simply have to add 1 line to /etc/default/grub and edit one entry in this file:
GRUB_DEFAULT=saved GRUB_SAVEDEFAULT=true
Adding GRUB_SAFEDEFAULT=true makes grub safe your last choice and GRUB_DEFAULT=saved makes it use your last choice. In most cases you find something like GRUB_DEFAULT=0 in your file.