* util/update-grub_lib.in: New file.
* DISTLIST: Add update-grub_lib.in.
* conf/common.rmk: Generate update-grub_lib and install it in
$(lib_DATA).
* Makefile.in: Add install routine for $(lib_DATA).
* util/grub.d/00_header.in: Use convert_system_path_to_grub_path()
function provided by update-grub_lib to support arbitrary paths of
unifont.pff.
* util/update-grub.in: Use convert_system_path_to_grub_path() to
initialize GRUB_DRIVE_BOOT and GRUB_DRIVE_BOOT_GRUB variables.
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