Modify "grub btrfs dirname" variable.

[Antynea]

Modify "GRUB_BTRFS_DIRNAME" variable

• Script / config file / grub-btrfs.service
  • 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.

[Antynea]

Warning
This should be used with caution.
If we use another folder than the default Grub installation folder generated during grub-install,
then it might break the use of the generated submenu by grub-btrfs.
Because

submenu '${submenuname}' ${protection_authorized_users}${unrestricted_access_submenu}{
	configfile "\${prefix}/grub-btrfs.cfg"
}

Will not be updated and will look for the file grub-btrfs.cfg in / grub.
${prefix} is a variable use by Grub in boot menu for path of grub folder.