Restoring previous layout of doxygen related files.

These files will undergo further re-organization in the coming
weeks. It is too early to fix on creating a subfolder for doxygen (as
the original pull request suggested).

doxygen/Makefile

@@ -1,12 +0,0 @@
-# Makefile used to generate the API documentation using Doxygen
-
-.PHONY: apidoc apidoc_clean
-
-all: apidoc
-
-apidoc:
-	mkdir -p apidoc
-	doxygen ble.doxyfile
-
-apidoc_clean:
-	rm -rf apidoc