Default GRUB configuration files

These files reside in the read-only ONIE-BOOT/grub directory.

grub.cfg -- The first GRUB config file read at boot time

If it detects the generated ONIE-CONFIG/grub.cfg it runs that file.

If not it runs the following in order:

  grub-common.cfg   - boiler plate stuff, default serial console
  grub-default.cfg  - sets default entry to "ONIE: OS Install"
  grub-onie.cfg     - ONIE menu entries and fallback

grub-common.cfg and grub-onie.cfg are the same files used by onie-grub-update.

The Linux serial console configuration is in it's own variable,
CMDLINE_LINUX_SERIAL.  NOS installers can use this variable in their
own GRUB menu entries.

By adding a grub.d/20_custom entry users could also change the serial console
configuration for all menuentries.  For example they could change the baud rate.