Makefile: Introduce  NON_STANDARD_IMAGES, ALL_IMAGES and DEFAULT_IMAGES vars
Makefile: Introduce  NON_STANDARD_IMAGES, ALL_IMAGES and DEFAULT_IMAGES vars

This commit also ensures that:
* all image targets are declared as PHONY
* "test" and not "tests" is declared as PHONY


To confirm that the Makefiles works as expected after this patch, the
list of executed commands before and after is compared.


Before:

```
make --dry-run > ../dockcross-make-baseline
make test --dry-run > ../dockcross-make-test-baseline
make browser-asmjs.test --dry-run > ../dockcross-make-browser-asmjs-test-baseline
make linux-ppc64le.test --dry-run > ../dockcross-make-linux-ppc64le-test-baseline
```

After:

```
make --dry-run > ../dockcross-make-current;
make test --dry-run > ../dockcross-make-test-current
make browser-asmjs.test --dry-run > ../dockcross-make-browser-asmjs-test-current
make linux-ppc64le.test --dry-run > ../dockcross-make-linux-ppc64le-test-current

for target in make make-test make-browser-asmjs-test make-linux-ppc64le-test; do
  diff --ignore-trailing-space ../dockcross-$target-current ../dockcross-$target-baseline > /dev/null 2>&1
  [[ $? == 1 ]] && \
    echo "" && \
    echo "Error: Problem with '${target}' target: Dry-run output before and after this commit do not match."
done
```

thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers,dockcross/dockcross,thewtex/cross-compilers,thewtex/cross-compilers