Makefile: Add implicit rule for STANDARD_IMAGES
Makefile: Add implicit rule for STANDARD_IMAGES

Adding a new images with the usual build rule:

  $(DOCKER) build -t $(ORG)/$@ $@

is now just a matter of adding an item to the "STANDARD_IMAGES" list.


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

Note that since this patch changes the order in which the images are
listed (manylinux is now last), the baseline and current files are
different. After inspection, we can confirm the difference is explained
by the order of the commands.

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