Added the native syslinux functions config_file(), ipappend_strs(), and reboot()...

Added the native syslinux functions config_file(), ipappend_strs(), and reboot() to Lua.c32. This allows the Lua script to query the config file name and the ipappend strings (pxelinux only), as well as to perform reboot (warm and cold) to the system.

In Lua.c32, the extension will be used as the following:
  1. syslinux.config_file() will return the config file string.
  2. syslinux.ipappend_strs() will return a table of IPAPPEND strings with numerical indices.
  3. syslinux.reboot() will perform cold reboot, while syslinux.reboot(1) will perform warm reboot.

Signed-off-by: Hung-chi Lihn <hlihn@google.com>
Signed-off-by: Erwan Velu <erwanaliasr1@gmail.com>