Fix the usage of CFG_xxxABLE_OPTIMIZE variables in Makefiles

Fix the usage of CFG_xxxABLE_OPTIMIZE variables in Makefiles

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.