build-sys: configurable debug shell tty path

configure.ac

@@ -754,6 +754,15 @@ AS_IF([test "x$enable_keymap" = "xyes"], [
 ])
 AM_CONDITIONAL([ENABLE_KEYMAP], [test "x$enable_keymap" = "xyes"])
 
+# ------------------------------------------------------------------------------
+debugshell=/dev/tty9
+AC_ARG_WITH([debugshelltty],
+        [AS_HELP_STRING([--with-debugshelltty=PATH],
+                [Specify the tty device for debug shell])],
+        [debugshell="$withval"],
+        [])
+AC_SUBST(debugshell)
+
 # ------------------------------------------------------------------------------
 have_manpages=no
 AC_ARG_ENABLE(manpages, AS_HELP_STRING([--disable-manpages], [disable manpages]))

units/debug-shell.service.in

@@ -6,7 +6,7 @@
 #  (at your option) any later version.
 
 [Unit]
-Description=Early root shell on tty9 FOR DEBUGGING ONLY
+Description=Early root shell on @debugshell@ FOR DEBUGGING ONLY
 Documentation=man:sushell(8)
 DefaultDependencies=no
 IgnoreOnIsolate=yes
@@ -17,7 +17,7 @@ ExecStart=@sushell@
 Restart=always
 RestartSec=0
 StandardInput=tty
-TTYPath=/dev/tty9
+TTYPath=@debugshell@
 TTYReset=yes
 TTYVHangup=yes
 KillMode=process