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