Add the lua scripts from the lua-bootloader SoC

These are the .lua files from from Pedro Souza's 2014 Summer of Code
project. Rui Paulo, Pedro Arthur and Wojciech A. Koszek also
contributed.

Obtained from: https://wiki.freebsd.org/SummerOfCode2014/LuaLoader
Sponsored by: Google Summer of Code

Improve the SoC lua menu code to bring it in line with forth
menu functionality

Submitted by: Zakary Nafziger
Sponsored by: FreeBSD Foundation

Use loader.setenv and loader.unsetenv instead of loader.perform

Convert from include("/boot/foo.lua") to foo = require("foo");
to bring in line with latest lua module conventions.

Enforce a uniform style for the new .lua files:
o hard tab indenation for 8 spaces
o don't have if foo then bar; else bas; end on one line

MFC After: 1 month
Relnotes: yes
Differential Review: https://reviews.freebsd.org/D14295