docs/Makefile: Default BUILDDIR based on MICROPY_PORT.

It doesn't make sense to duplicate both on command line, and MICROPY_PORT
is effectively mandatory to build docs.

docs/Makefile

@@ -5,7 +5,7 @@
 SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 PAPER         =
-BUILDDIR      = build
+BUILDDIR      = build/$(MICROPY_PORT)
 
 # User-friendly check for sphinx-build
 ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)

docs/README.md

@@ -21,7 +21,7 @@ preferably in a virtualenv:
 
 In `micropython/docs`, build the docs:
 
-    make MICROPY_PORT=<port_name> BUILDDIR=build/<port_name> html
+    make MICROPY_PORT=<port_name> html
 
 Where `<port_name>` can be `unix`, `pyboard`, `wipy` or `esp8266`.