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* util/update-grub.in (GRUB_GFXMODE): Export variable.
* util/grub.d/00_header.in: Allow the administrator to change default
gfxmode via ${GRUB_GFXMODE}.
All executable files in this directory are processed in shell expansion order. 00_*: Reserved for 00_header. 10_*: Native boot entries. 20_*: Third party apps (e.g. memtest86+). The number namespace in-between is configurable by system installer and/or administrator. For example, you can add an entry to boot another OS as 01_otheros, 11_otheros, etc, depending on the position you want it to occupy in the menu; and then adjust the default setting via /etc/default/grub.