[tlbuild] LuaMetaTeX for TeX Live

[Karl Berry]

    way: would the best location for the sources then be "texk/luametatex"
    (which is where lcdf-typetools and alike live)? And if we don't manage
    to figure it out: just stick with "utils"?

The distinction between utils and texk is that programs in texk use
kpathsea, and programs in utils do not. Thus lmtx should be in utils.

Unless I'm forgetting something, asy is the *only* program in the entire
source tree which is not handled with the top-level configure. Let's not
add another one. -k

P.S. Regarding adding a new normal program or package to the source
tree, peb and I wrote a lot of information in the tlbuild manual. FYI.