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