kboot: Create quit command

Create a command that just quits out of the loader. Useful for
debugging.

Sponsored by:		Netflix
Reviewed by:		tsoome
Differential Revision:	https://reviews.freebsd.org/D50589

stand/kboot/kboot/main.c

@@ -655,3 +655,15 @@ command_fdt(int argc, char *argv[])
 
 COMMAND_SET(fdt, "fdt", "flattened device tree handling", command_fdt);
 #endif
+
+/*
+ * Support quitting.
+ */
+static int
+command_quit(int argc, char *argv[])
+{
+	exit(0);
+	return (CMD_OK);
+}
+
+COMMAND_SET(quit, "quit", "exit the program", command_quit);