Makefile: Remove dependencies on a non-generated files
Makefile: Remove dependencies on a non-generated files

Since image targets are all "phony", they are always consider out-of-date
and will always be rebuilt. This means there is no need to add an explicit
dependency on non-generated files like  "Dockerfile", "settings.mk" and
"test/run.py".

The only required dependencies on Dockerfile are the one for the "base"
and "manylinux" images.

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
```

After:

```
make --dry-run > ../dockcross-make-current;
make test --dry-run > ../dockcross-make-test-current

for target in make make-test; do
  diff ../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,thewtex/cross-compilers,dockcross/dockcross,thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers