Some minor configure and Makefile cleanup.

Move CFG_TARGET_TRIPLES to CFG_TARGET, since only one target is
supported anyway.

Create config.{mk,tmp,status} files in build directory instead of the
source directory.
Change how backup/restore make task works.

Now we write $SRC/.config.mk.last to store the most recent configure
variables for the backup and restore task. Also, they no longer fail if
they can't find the variables, and will skip the backup/restore stages
and keep going.
auto merge of #2232 : metajack/servo/configure-cleanup, r=jdm

Move CFG_TARGET_TRIPLES to CFG_TARGET, since only one target is
supported anyway.

Create config.{mk,tmp,status} files in build directory instead of the
source directory.

r? @larsbergstrom
Remove C++-specific union codegen code.
Reuse getTypes in UnionConversions().

This should be equivalent, except that the removed code ignores return
values (which we currently don't support).
Merge UnionConversions() into UnionTypes().

It's not very useful to have one but not the other, so it makes more sense
to deal with them together.
auto merge of #2268 : Ms2ger/servo/unions, r=jdm