Finit bringup: add optional fallback shell

This patch adds an `--enable-fallback-shell` option to confiugre.
When enabled Finit will start `/bin/sh` on the console, but only
if there are no active TTYs running in the current runlevel.

With this it will be easier to do board bringup with Finit, but it also
enables using the fallback shell in a rescue mode (runlevel) where no
TTYs are allowed to run.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

configure.ac

@@ -41,6 +41,9 @@ AC_ARG_ENABLE(emergency_shell,
 	AS_HELP_STRING([--enable-emergency-shell], [Experimental emergency fallback at boot.
 	                Only use this for debugging very early boot problems with Finit.]))
 
+AC_ARG_ENABLE(fallback_shell,
+        AS_HELP_STRING([--enable-fallback-shell], [Start a shell if no TTYs can start.]))
+
 AC_ARG_ENABLE(rw-rootfs,
         AS_HELP_STRING([--enable-rw-rootfs], [Remount / R/W at boot, usually not for embedded]))
 
@@ -120,6 +123,9 @@ AC_ARG_WITH(random-seed,
 AS_IF([test "x$enable_emergency_shell" = "xyes"], [
         AC_DEFINE(EMERGENCY_SHELL,  1, [Fork off init and let PID 1 wait and start /bin/sh if init crashes.])])
 
+AS_IF([test "x$enable_fallback_shell" = "xyes"], [
+        AC_DEFINE(FALLBACK_SHELL,  1, [Start /bin/sh on console if no TTYs.])])
+
 AS_IF([test "x$enable_rw_rootfs" = "xyes"], [
         AC_DEFINE(REMOUNT_ROOTFS,  1, [Remount / as rw at boot])])