docs: filesystems: convert efivarfs.txt to ReST

Trivial changes:

- Add a SPDX header;
- Adjust document title;
- Mark a literal block as such;
- Add it to filesystems/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/215691d747055c4ccb038ec7d78d8d1fe87fe2c0.1581955849.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/efivarfs.txt → Documentation/filesystems/efivarfs.rst

+.. SPDX-License-Identifier: GPL-2.0
 
+=======================================
 efivarfs - a (U)EFI variable filesystem
+=======================================
 
 The efivarfs filesystem was created to address the shortcomings of
 using entries in sysfs to maintain EFI variables. The old sysfs EFI
@@ -11,7 +14,7 @@ than a single page, sysfs isn't the best interface for this.
 Variables can be created, deleted and modified with the efivarfs
 filesystem.
 
-efivarfs is typically mounted like this,
+efivarfs is typically mounted like this::
 
 	mount -t efivarfs none /sys/firmware/efi/efivars
 

Documentation/filesystems/index.rst

@@ -60,6 +60,7 @@ Documentation for filesystem implementations.
    debugfs
    dlmfs
    ecryptfs
+   efivarfs
    fuse
    overlayfs
    virtiofs