README: Update "Dependencies" section.

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.

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