Move sh selection out of GUI makefile

* The TWRP_SH_TARGET variable is only used if TW_USE_TOOLBOX == false,
  so it doesn't really need to be defined.
* Move the busybox sh symlink creation nearer to the toolbox/busybox
  build rules and take advantage of LOCAL_POST_INSTALL_CMD which has
  been around since Android 4.3.

Change-Id: I7850f9c3e113d43ad519dd5c9a7d25e36ec1c426
diff --git a/Android.mk b/Android.mk
index 1a4b9c0..8189ffa 100644
--- a/Android.mk
+++ b/Android.mk
@@ -380,6 +380,11 @@
 endif
 ifneq ($(TW_USE_TOOLBOX), true)
     LOCAL_ADDITIONAL_DEPENDENCIES += busybox_symlinks
+    ifeq ($(shell test $(PLATFORM_SDK_VERSION) -lt 24; echo $$?),0)
+        LOCAL_POST_INSTALL_CMD := \
+            $(hide) mkdir -p $(TARGET_RECOVERY_ROOT_OUT)/sbin && \
+            ln -sf /sbin/busybox $(TARGET_RECOVERY_ROOT_OUT)/sbin/sh
+    endif
 else
     ifneq ($(wildcard external/toybox/Android.mk),)
         LOCAL_ADDITIONAL_DEPENDENCIES += toybox_symlinks
@@ -391,6 +396,7 @@
         LOCAL_ADDITIONAL_DEPENDENCIES += unzip
     endif
 endif
+
 ifneq ($(TW_NO_EXFAT), true)
     LOCAL_ADDITIONAL_DEPENDENCIES += mkexfatfs fsckexfat
     ifneq ($(TW_NO_EXFAT_FUSE), true)