busybox: Allow running from within the Nix store

This enables running the busybox program directly from the Nix store
(i.e. as /nix/store/<hash>-busybox). We need this for the stdenv
bootstrap.

pkgs/os-specific/linux/busybox/busybox-in-store.patch

@@ -0,0 +1,16 @@
+Allow BusyBox to be invoked as "<something>-busybox". This is
+necessary when it's run from the Nix store as <hash>-busybox during
+stdenv bootstrap.
+
+diff -ru -x '*~' busybox-1.22.1-orig/libbb/appletlib.c busybox-1.22.1/libbb/appletlib.c
+--- busybox-1.22.1-orig/libbb/appletlib.c	2014-01-09 19:15:44.000000000 +0100
++++ busybox-1.22.1/libbb/appletlib.c	2014-10-29 09:53:01.232052068 +0100
+@@ -764,7 +764,7 @@
+ 	int applet = find_applet_by_name(name);
+ 	if (applet >= 0)
+ 		run_applet_no_and_exit(applet, argv);
+-	if (strncmp(name, "busybox", 7) == 0)
++	if (strstr(name, "busybox") != 0)
+ 		exit(busybox_main(argv));
+ }
+ 

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

@@ -28,6 +28,8 @@ stdenv.mkDerivation rec {
     sha256 = "12v7nri79v8gns3inmz4k24q7pcnwi00hybs0wddfkcy1afh42xf";
   };
 
+  patches = [ ./busybox-in-store.patch ];
+
   configurePhase = ''
     export KCONFIG_NOTIMESTAMP=1
     make ${if enableMinimal then "allnoconfig" else "defconfig"}