busybox: add enableAppletSymlinks?true

This commit adds an argument enableAppletSymlinks?true, which can be
set to false in order to turn off CONFIG_INSTALL_APPLET_SYMLINKS by
users if they only want the main busybox binary in their profile.
This is particularly useful when building pkgsStatic.busybox.

pkgs/os-specific/linux/busybox/default.nix

@@ -1,6 +1,7 @@
 { stdenv, lib, buildPackages, fetchurl, fetchFromGitLab
 , enableStatic ? stdenv.hostPlatform.isStatic
 , enableMinimal ? false
+, enableAppletSymlinks ? true
 # Allow forcing musl without switching stdenv itself, e.g. for our bootstrapping:
 # nix build -f pkgs/top-level/release.nix stdenvBootstrapTools.x86_64-linux.dist
 , useMusl ? stdenv.hostPlatform.libc == "musl", musl
@@ -95,6 +96,11 @@ stdenv.mkDerivation rec {
       CONFIG_STATIC y
     ''}
 
+    ${lib.optionalString (!enableAppletSymlinks) ''
+      CONFIG_INSTALL_APPLET_DONT y
+      CONFIG_INSTALL_APPLET_SYMLINKS n
+    ''}
+
     # Use the external mount.cifs program.
     CONFIG_FEATURE_MOUNT_CIFS n
     CONFIG_FEATURE_MOUNT_HELPERS y