lualoader: Implement try_include and use it for including the local module

This provides a way to optionally include a module without having to wrap it
in filesystem checks. try_include is a little more robust, using the lua
search path instead of forcing us to explicitly consider all of the places
we could want to include a module. Errors are still generally raised from
trying to load the module, but ENOENT will not get raised unless we're doing
a verbose load.

This will also be used to split out logo/brand graphics into their own files
so that we can safely scale up the number of graphics included without
worrying about the extra memory consumption- opting to lazily load graphics
instead.

Reviewed by:	cem
Differential Revision:	https://reviews.freebsd.org/D14658

stand/lua/core.lua

@@ -41,6 +41,26 @@ local function composeLoaderCmd(cmd_name, argstr)
 	return cmd_name
 end
 
+-- Globals
+-- try_include will return the loaded module on success, or nil on failure.
+-- A message will also be printed on failure, with one exception: non-verbose
+-- loading will suppress 'module not found' errors.
+function try_include(module)
+	local status, ret = pcall(require, module)
+	-- ret is the module if we succeeded.
+	if status then
+		return ret
+	end
+	-- Otherwise, ret is just a message; filter out ENOENT unless we're
+	-- doing a verbose load. As a consequence, try_include prior to loading
+	-- configuration will not display 'module not found'. All other errors
+	-- in loading will be printed.
+	if config.verbose or ret:match("^module .+ not found") == nil then
+		print(ret)
+	end
+	return nil
+end
+
 -- Module exports
 -- Commonly appearing constants
 core.KEY_BACKSPACE	= 8

stand/lua/loader.lua

@@ -43,11 +43,7 @@ if not core.isMenuSkipped() then
 end
 local password = require("password")
 
-local result = lfs.attributes("/boot/lua/local.lua")
--- Effectively discard any errors; we'll just act if it succeeds.
-if result ~= nil then
-	require("local")
-end
+try_include("local")
 
 config.load()
 if core.isUEFIBoot() then