mksh: add mkshrc when using TW_USE_TOOLBOX

Fixes broken shell prompt in Terminal and adb shell

Before: https://del.dog/7vphlkc
After: https://del.dog/vhkjgo8

Add as prebuilt with suffix "_twrp" to avoid conflict
with original mkshrc and rename via POST_INSTALL_CMD

Prebuilt version adjusts TMPDIR variable from
"/data/local/tmp" to "/tmp" for TWRP

Change-Id: I42a1b2a8c114f66dbe659c07485f1dfae8a5d314
diff --git a/prebuilt/mkshrc_twrp b/prebuilt/mkshrc_twrp
new file mode 100755
index 0000000..c103d6c
--- /dev/null
+++ b/prebuilt/mkshrc_twrp
@@ -0,0 +1,21 @@
+# Copyright (c) 2010, 2012, 2013, 2014
+#	Thorsten Glaser <tg@mirbsd.org>
+# This file is provided under the same terms as mksh.
+#-
+# Minimal /system/etc/mkshrc for Android
+#
+# Support: https://launchpad.net/mksh
+
+: ${HOSTNAME:=$(getprop ro.product.device)}
+: ${HOSTNAME:=android}
+: ${TMPDIR:=/tmp}
+export HOSTNAME TMPDIR
+
+if (( USER_ID )); then PS1='$'; else PS1='#'; fi
+PS4='[$EPOCHREALTIME] '; PS1='${|
+	local e=$?
+
+	(( e )) && REPLY+="$e|"
+
+	return $e
+}$HOSTNAME:${PWD:-?} '"$PS1 "