loader: Add loader.exit

Add loader.exit(status). While one can get alomst this behavior with
loader.perform("quit"), quit doesn't allow a value to be returned to the
firmware. The interpretation of 'status' is firmware specific. This can
be used when autobooting doesn't work in scripts, for example, to allow
the firmware to try something else...

Sponsored by:		Netflix

Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D44094

stand/liblua/lutils.c

@@ -109,6 +109,13 @@ lua_perform(lua_State *L)
 	return 1;
 }
 
+static int
+lua_exit(lua_State *L)
+{
+	exit(luaL_checkinteger(L, 1));
+	return 0;
+}
+
 static int
 lua_command_error(lua_State *L)
 {
@@ -380,14 +387,15 @@ lua_writefile(lua_State *L)
 
 #define REG_SIMPLE(n)	{ #n, lua_ ## n }
 static const struct luaL_Reg loaderlib[] = {
-	REG_SIMPLE(delay),
-	REG_SIMPLE(command_error),
 	REG_SIMPLE(command),
-	REG_SIMPLE(interpret),
-	REG_SIMPLE(parse),
+	REG_SIMPLE(command_error),
+	REG_SIMPLE(delay),
+	REG_SIMPLE(exit),
 	REG_SIMPLE(getenv),
 	REG_SIMPLE(has_command),
 	REG_SIMPLE(has_feature),
+	REG_SIMPLE(interpret),
+	REG_SIMPLE(parse),
 	REG_SIMPLE(perform),
 	REG_SIMPLE(printc),	/* Also registered as the global 'printc' */
 	REG_SIMPLE(setenv),

stand/lua/loader.lua.8

@@ -61,6 +61,10 @@ Returns the error string from the last command to fail.
 Like
 .Fn perform
 but the arguments are already parsed onto the stack.
+.It Fn exit status
+Exit the boot loader back to the firmware with a status of
+.Va status .
+The interpretation of this value is firmware specific.
 .It Fn interpret str
 Execute the loader builtin command
 .Va str