l3build: move default generated folders under common build/

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5077 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: allow the ctan TDS to be customised

e.g., if you want to install into "generic/bundle/module",
can write

    tdsroot = "generic"

not sure if this name is best; happy with alternatives.
Maybe "installdir"?

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5078 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: cfg files for both 2e and l3

(not sure how they are installed as part of our build system...)

* the message at the top of the tlg file is now customised to
  allow correct output for both l3 and 2e.

* \InputIfFileExists implemented quickly in plain if necessary

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5079 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: continue documentation

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5080 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: unpackdir needs generating + cleaning if necessary

(again not sure if this is the place to put this but seems
better than my last attempt of subverting cp()!)

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5081 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: add engines to documented variables

(not sure i missed these)

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5082 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: build test files updated

* at present, can't test with multiple engines under plain
  (to be considered)
  but one engine at a time works, as shown here

* cfg files work under both latex and plain

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5083 de43f980-851b-0410-b2f7-c40aca1f87e0