README: Update "Dependencies" section.

pfalcon · pfalcon · commit 6e06512e0ff4 · 2017-09-11T00:33:39.000+03:00
Given that various ports now require submodules, rewrite the section
to be more generic.

Also, add git submodule update command to other sections for easy user
start.
diff --git a/README.md b/README.md
@@ -72,6 +72,7 @@ Alternatively, fallback implementation based on setjmp/longjmp can be used.
 
 To build (see section below for required dependencies):
 
+    $ git submodule update --init
     $ cd ports/unix
     $ make axtls
     $ make
@@ -104,32 +105,36 @@ Standard library modules come from
 External dependencies
 ---------------------
 
-Building Unix version requires some dependencies installed. For
+Building MicroPython ports may require some dependencies installed.
+
+For Unix port, `libffi` library and `pkg-config` tool are required. On
 Debian/Ubuntu/Mint derivative Linux distros, install `build-essential`
 (includes toolchain and make), `libffi-dev`, and `pkg-config` packages.
 
-Other dependencies can be built together with MicroPython. Oftentimes,
-you need to do this to enable extra features or capabilities. To build
+Other dependencies can be built together with MicroPython. This may
+be required to enable extra features or capabilities, and in recent
+versions of MicroPython, these may be enabled by default. To build
 these additional dependencies, first fetch git submodules for them:
 
     $ git submodule update --init
 
-Use this same command to get the latest versions of dependencies, as
-they are updated from time to time. After that, in `ports/unix/` dir, execute:
+Use the same command to get the latest versions of dependencies, as
+they are updated from time to time. After that, in the port directory
+(e.g. `ports/unix/`), execute:
 
     $ make deplibs
 
 This will build all available dependencies (regardless whether they
 are used or not). If you intend to build MicroPython with additional
 options (like cross-compiling), the same set of options should be passed
-to `make deplibs`. To actually enabled use of dependencies, edit
+to `make deplibs`. To actually enable/disable use of dependencies, edit
 `ports/unix/mpconfigport.mk` file, which has inline descriptions of the options.
-For example, to build SSL module (required for `upip` tool described above),
-set `MICROPY_PY_USSL` to 1.
+For example, to build SSL module (required for `upip` tool described above,
+and so enabled by dfeault), `MICROPY_PY_USSL` should be set to 1.
 
-In `ports/unix/mpconfigport.mk`, you can also disable some dependencies enabled
-by default, like FFI support, which requires libffi development files to
-be installed.
+For some ports, building required dependences is transparent, and happens
+automatically. They still need to be fetched with the git submodule command
+above.
 
 The STM32 version
 -----------------
@@ -141,6 +146,7 @@ https://launchpad.net/gcc-arm-embedded
 
 To build:
 
+    $ git submodule update --init
     $ cd ports/stm32
     $ make
 
