grub hotkey

grub entries are generated by the scripts in /etc/grub.d/ and collected as /boot/grub/grub.cfg
the overall configuration is in /etc/default/grub
you can edit the script /etc/default/grub/40_custom
by copying entries from  /boot/grub/grub.cfg
to add an hotkey,

menuentry ... --hotkey w {
...


sources:
https://linuxnorth.wordpress.com/2011/03/09/grub2-revisited/
https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1178618