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.
auto merge of #2232 : metajack/servo/configure-cleanup, r=larsbergstrom

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