Auto merge of #2047 - akiss77:makefile-cfg-optimize, r=alexcrichton

Until now, there was a CFG_DISABLE_OPTIMIZE that controlled whether `--release` is passed to cargo during the build, and a CFG_ENABLE_OPTIMIZE that controlled where `make install` (among others) looked for the already built cargo executable. Unfortunately, if none of these were specified, `make all` built a release cargo but `make install` looked for it under `$(CFG_TARGET)/debug` (and aborted with "Please run `make` first"). This patch keeps CFG_DISABLE_OPTIMIZE only and uses it consistently.