Do not limit file copying to only grub.cfg and instead copy all GRUB configuration files and assets to both the ISO9660 and FAT image.
This will allow for including custom images, fonts, etc.

To easily match all non-configuration files (i.e. files without the .cfg extension), bash's extended glob feature will be enabled.
Actions common to multiple _make_bootmode_uefi-*.grub are split off into dedicated functions:

* _make_common_bootmode_grub_copy_to_efibootimg,
* _make_common_bootmode_grub_copy_to_isofs,
* _make_common_bootmode_grub_cfg.

Use the same du command in all efiboot_imgsize variable assignments.

Fixes #185.