stand/lua: Reduce magic numbers

Enter/backspace values are hardcoded in both the menu and password scripts.
Separate these out to core for reuse between the two.

stand/lua/core.lua

@@ -28,6 +28,10 @@
 
 local core = {};
 
+-- Commonly appearing constants
+core.KEY_ENTER = 13
+core.KEY_BACKSPACE = 127
+
 function core.setVerbose(b)
 	if (b == nil) then
 		b = not core.verbose;

stand/lua/menu.lua

@@ -273,9 +273,9 @@ function menu.run(m)
 		local key = io.getchar();
 
 		-- Special key behaviors
-		if (key == 127) and (m ~= menu.welcome) then
+		if (key == core.KEY_BACKSPACE) and (m ~= menu.welcome) then
 			break
-		elseif (key == 13) then
+		elseif (key == core.KEY_ENTER) then
 			core.boot();
 			-- Should not return
 		end
@@ -357,7 +357,7 @@ function menu.autoboot()
 		screen.defcursor();
 		if io.ischar() then
 			local ch = io.getchar();
-			if ch == 13 then
+			if ch == core.KEY_ENTER then
 				break;
 			else
 				-- prevent autoboot when escaping to interpreter

stand/lua/password.lua

@@ -37,11 +37,11 @@ function password.read()
 
 	repeat
 		ch = io.getchar();
-		if ch == 13 then
+		if ch == core.KEY_ENTER then
 			break;
 		end
 
-		if ch == 8 then
+		if ch == core.KEY_BACKSPACE then
 			if n > 0 then
 				n = n - 1;
 				-- loader.printc("\008 \008");