grub-btrfs.service: remove unnecessary path for bash and grub-mkconfig command
readme.md: add warning section to systemd service section
config file: Improves the description of the variable GRUB_BTRFS_MKCONFIG
* Modify "GRUB_BTRFS_DIRNAME"
* Full path to Grub folder is now configurable (/boot is no longer hard coded).
* Detection of the boot partition is now based on the location of the Grub folder.
* Warning [see](https://github.com/Antynea/grub-btrfs/pull/113#issuecomment-705916729)
* Make 'grub-mkconfig' command name configurable
- Use 'grub-mkconfig' as the default
- Note that some systems use 'grub2-mkconfig'
- Update 10-update_grub.conf to use the configurable grub directory name
- Refactor the systemd commands for clarity and to avoid duplicate regeneration on first run
* Update check_uuid_required() to work for partial regeneration of the GRUB menu