Motivation for this change

Without CONFIG_MOUSE_ELAN_I2C_SMBUS, the kernel warns

psmouse serio1: elantech: The touchpad can support a better bus than the too old PS/2 protocol. Make sure MOUSE_PS2_ELANTECH_SMBUS and MOUSE_ELAN_I2C_SMBUS are enabled to get a better touchpad experience.

and it is quite correct, at least on this laptop (ThinkPad Yoga 14 20FY), where CONFIG_MOUSE_ELAN_I2C_SMBUS=y in conjunction with boot.extraModprobeConfig="options psmouse elantech_smbus=1" are necessary to eliminate an annoying sticky touchpad button issue.

CONFIG_MOUSE_ELAN_I2C_SMBUS is enabled in Arch, CentOS, Fedora, and Ubuntu. (Debian lacks its prerequisite CONFIG_MOUSE_ELAN_I2C entirely.)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.