stand: Print a warning of the loader is too old

If the loader is < 3.0, print a warning that it's too old and needs to
be upgraded.

Sponsored by:		Netflix
Reviewed by:		kevans
Differential Revision:	https://reviews.freebsd.org/D45889

stand/lua/core.lua

@@ -542,6 +542,27 @@ function core.nextConsoleChoice()
 	end
 end
 
+-- Sanity check the boot loader revision
+-- Loaders with version 3.0 have everything that we need without backwards
+-- compatible hacks. Warn users that still have old versions to upgrade so
+-- that we can remove the backwards compatible hacks in the future since
+-- they have been there a long time.
+local loader_major = 3
+
+function core.loaderTooOld()
+	return loader.version == nil or loader.version < loader_major * 1000
+end
+
+if core.loaderTooOld() then
+	print("**********************************************************************")
+	print("**********************************************************************")
+	print("*****                                                            *****")
+	print("*****           BOOT LOADER IS TOO OLD. PLEASE UPGRADE.          *****")
+	print("*****                                                            *****")
+	print("**********************************************************************")
+	print("**********************************************************************")
+end
+
 recordDefaults()
 hook.register("config.reloaded", core.clearCachedKernels)
 return core

stand/lua/core.lua.8

@@ -218,6 +218,10 @@ If there are no elements, this returns nil and nil.
 If there is one element, this returns the front element and an empty table.
 This will not operate on truly associative tables; numeric indices are
 required.
+.It Fn core.loaderTooOld
+Returns true if the loader is too old.
+Specifically, this means, is the loader old enough to require one or more
+workarounds in the current lua base modules.
 .El
 .Sh SEE ALSO
 .Xr loader.conf 5 ,