Makefile: Add comments and move implicit rules at the end of the file

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 --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
```

Makefile

@@ -1,27 +1,40 @@
+
+#
+# Parameters
+#
+
+# Name of the docker executable
 DOCKER = docker
+
+# Docker organization to pull the images from
 ORG = dockcross
+
+# Directory where to generate the dockcross script for each images (e.g bin/dockcross-manylinux-x64)
 BIN = bin
 
+# These images are built using the "build implicit rule"
 STANDARD_IMAGES = android-arm linux-x86 linux-x64 linux-arm64 linux-armv5 linux-armv6 linux-armv7 windows-x86 windows-x64
 
+# These images are associated with the 'images' target
 IMAGES = $(STANDARD_IMAGES) manylinux-x64 manylinux-x86
 
-# Arguments for test/run.py associated with standard images
+# Optional arguments for test runner (test/run.py) associated with "testing implicit rule"
 windows-x86.test_ARGS = --exe-suffix ".exe"
 windows-x64.test_ARGS = --exe-suffix ".exe"
 
+#
+# images: This target builds all IMAGES (because it is the first one, it is built by default)
+#
 images: base $(IMAGES)
 
+#
+# test: This target ensures all IMAGES are built and run the associated tests
+#
 test: base.test $(addsuffix .test,$(IMAGES))
 
-$(STANDARD_IMAGES): base
+#
-	$(DOCKER) build -t $(ORG)/$@ $@
+# browser-asmjs
-
+#
-.SECONDEXPANSION:
-$(addsuffix .test,$(STANDARD_IMAGES)): $$(basename $$@)
-	$(DOCKER) run --rm dockcross/$(basename $@) > $(BIN)/dockcross-$(basename $@) && chmod +x $(BIN)/dockcross-$(basename $@)
-	$(BIN)/dockcross-$(basename $@) python test/run.py $($@_ARGS)
-
 browser-asmjs: base
 	cp -r test browser-asmjs/
 	$(DOCKER) build -t $(ORG)/browser-asmjs browser-asmjs
@@ -31,6 +44,9 @@ browser-asmjs.test: browser-asmjs
 	$(DOCKER) run --rm dockcross/browser-asmjs > $(BIN)/dockcross-browser-asmjs && chmod +x $(BIN)/dockcross-browser-asmjs
 	$(BIN)/dockcross-browser-asmjs python test/run.py --exe-suffix ".js"
 
+#
+# linux-ppc64le
+#
 linux-ppc64le: base
 	$(DOCKER) build -t $(ORG)/linux-ppc64le linux-ppc64le
 
@@ -38,6 +54,9 @@ linux-ppc64le.test: linux-ppc64le
 	$(DOCKER) run --rm dockcross/linux-ppc64le > $(BIN)/dockcross-linux-ppc64le && chmod +x $(BIN)/dockcross-linux-ppc64le
 	$(BIN)/dockcross-linux-ppc64le python test/run.py --languages C
 
+#
+# manylinux-x64
+#
 manylinux-x64/Dockerfile: manylinux-x64/Dockerfile.in common.docker
 	sed '/common.docker/ r common.docker' manylinux-x64/Dockerfile.in > manylinux-x64/Dockerfile
 
@@ -48,6 +67,9 @@ manylinux-x64.test: manylinux-x64
 	$(DOCKER) run --rm dockcross/manylinux-x64 > $(BIN)/dockcross-manylinux-x64 && chmod +x $(BIN)/dockcross-manylinux-x64
 	$(BIN)/dockcross-manylinux-x64 /opt/python/cp35-cp35m/bin/python test/run.py
 
+#
+# manylinux-x86
+#
 manylinux-x86/Dockerfile: manylinux-x86/Dockerfile.in common.docker
 	sed '/common.docker/ r common.docker' manylinux-x86/Dockerfile.in > manylinux-x86/Dockerfile
 
@@ -58,6 +80,9 @@ manylinux-x86.test: manylinux-x86
 	$(DOCKER) run --rm dockcross/manylinux-x86 > $(BIN)/dockcross-manylinux-x86 && chmod +x $(BIN)/dockcross-manylinux-x86
 	$(BIN)/dockcross-manylinux-x86 /opt/python/cp35-cp35m/bin/python test/run.py
 
+#
+# base
+#
 Dockerfile: Dockerfile.in common.docker
 	sed '/common.docker/ r common.docker' Dockerfile.in > Dockerfile
 
@@ -68,4 +93,18 @@ base.test: base
 	mkdir -p $(BIN)
 	$(DOCKER) run --rm dockcross/base > $(BIN)/dockcross-base && chmod +x $(BIN)/dockcross-base
 
+#
+# build implicit rule
+#
+$(STANDARD_IMAGES): base
+	$(DOCKER) build -t $(ORG)/$@ $@
+
+#
+# testing implicit rule
+#
+.SECONDEXPANSION:
+$(addsuffix .test,$(STANDARD_IMAGES)): $$(basename $$@)
+	$(DOCKER) run --rm dockcross/$(basename $@) > $(BIN)/dockcross-$(basename $@) && chmod +x $(BIN)/dockcross-$(basename $@)
+	$(BIN)/dockcross-$(basename $@) python test/run.py $($@_ARGS)
+
 .PHONY: base images $(IMAGES) tests %.test