busybox: Add a default udhcpc dispatcher script

The udhcpc binary which currently ships as part of the busybox
derivation will by default search for a dispatcher script at the
location /usr/share/udhcpc/default.script.

This commit includes a working default script with udhcpc and updates
the location where udhcpc searches for this script.

The script was taken the script from the udhcpc package in debian
buster. The only changes from that script is to make it use paths from
the nix store and remove the run-time check for /sbin/resolvconf.

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

@@ -80,6 +80,9 @@ stdenv.mkDerivation rec {
     # Bump from 4KB, much faster I/O
     CONFIG_FEATURE_COPYBUF_KB 64
 
+    # Set the path for the udhcpc script
+    CONFIG_UDHCPC_DEFAULT_SCRIPT "$out/share/default.script"
+
     ${extraConfig}
     CONFIG_CROSS_COMPILER_PREFIX "${stdenv.cc.targetPrefix}"
     ${libcConfig}
@@ -94,6 +97,12 @@ stdenv.mkDerivation rec {
     makeFlagsArray+=("CC=${stdenv.cc.targetPrefix}cc -isystem ${musl.dev}/include -B${musl}/lib -L${musl}/lib")
   '';
 
+  postInstall = ''
+    mkdir $out/share
+    substituteAll ${./default.script} $out/share/default.script
+    chmod +x $out/share/default.script
+  '';
+
   depsBuildBuild = [ buildPackages.stdenv.cc ];
 
   buildInputs = lib.optionals (enableStatic && !useMusl && stdenv.cc.libc ? static) [ stdenv.cc.libc stdenv.cc.libc.static ];