mstflint: add description
mstflint: add description

While I am aware of a new version 4.15.0-1, I avoided touching the
package itself as I can not test the package with real hardware.

Signed-off-by: Masanori Ogino <9fd08c6158f4e2c4695b3924e22cb1a790721236@users.noreply.github.com>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs