Added libopencm3 as submodule for Cortex-M port

- removed instructions on adding submodule from README.md and Makefile
- submodule libopencm3 at commit 27b826bc4a09345f63d3b8b5fcd3cb9f145cd1a5

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "ports/cortex-m/libopencm3"]
+	path = ports/cortex-m/libopencm3
+	url = https://github.com/libopencm3/libopencm3.git

ports/cortex-m/Makefile

@@ -217,7 +217,6 @@ $(LIB_DIR)/lib$(LIBNAME).a:
 		printf "######## ERROR ########\n"; \
 		printf "\tlibopencm3 is not initialized.\n"; \
 		printf "\tPlease run:\n"; \
-		printf "\t$$ git submodule add https://github.com/libopencm3/libopencm3.git\n"; \
 		printf "\t$$ git submodule init\n"; \
 		printf "\t$$ git submodule update\n"; \
 		printf "\tbefore running make.\n"; \

ports/cortex-m/README.md

@@ -52,17 +52,9 @@ which will be removed by `make clean`.
 Unless you decide to use an external installation of libopencm3, you will have
 to set up the libopencm3 sub-module:
 ```
-git submodule add https://github.com/libopencm3/libopencm3.git
 git submodule init
 git submodule update
 ```
-Optional: As of 2015-07-08 the libopencm3 API has not been declared stable. If
-future changes break the build, you can check out the older revision used while
-developing this port:
-```
-cd libopencm3
-git checkout a4bb8f7e240c9f238384cf86d009002ba42a25ed
-```
 
 ## Building and Flashing
 To build the test suite, run