feat: add auto-detection for Fedora distribution

[reynegton]

Fedora uses non-standard paths and command names for GRUB (e.g., /boot/grub2 instead of /boot/grub, and grub2-mkconfig). This change adds a conditional block to detect Fedora via /etc/fedora-release and automatically sets the appropriate variables, improving the out-of-the-box experience for Fedora users.

[reynegton]

Hi @Antynea,

I’m opening this PR because I recently tried to set up grub-btrfs on Fedora and realized it doesn't work out-of-the-box. Due to Fedora's specific
naming conventions and directory structure, the script fails to locate the necessary GRUB files and commands by default.

With this patch, grub-btrfs becomes 100% "plug and play" for Fedora users.

Detailed changes and reasoning:
The PR adds an auto-detection block that triggers when /etc/fedora-release is detected. It automatically sets the following variables to match
Fedora's standards:

  GRUB_BTRFS_GRUB_DIRNAME & GRUB_BTRFS_GBTRFS_DIRNAME*: Changed to /boot/grub2. Unlike many other distros that use /boot/grub, Fedora explicitly

uses the grub2 suffix for its boot directory.
GRUB_BTRFS_MKCONFIG*: Changed to grub2-mkconfig. Fedora does not provide a grub-mkconfig binary; it uses the grub2- prefixed version.
GRUB_BTRFS_SCRIPT_CHECK*: Changed to grub2-script-check. Similar to the above, this follows Fedora's command naming scheme.

Before these changes, a Fedora user would have to manually investigate why the snapshots weren't appearing and then edit the config file. This
update removes that friction entirely.

I've verified this on Fedora 44 and confirmed that the snapshots are now correctly generated and added to the menu without any manual
intervention.

Let me know if you have any questions!

[Antynea]

Hi @reynegton,
That’s right, Fedora uses specific naming conventions and directory structures that haven’t changed for several versions.
The Makefile also includes detection for Fedora, but the config file needs to be edited manually.

Thanks for your time, I’ll merge your PR.

[reynegton]

thanks!