Make building cli, ncp, and examples optional. (#132)

By default, only core is built.  Added the following configure options:
--enable-cli
--enable-ncp
--with-examples=TARGET

examples/Makefile.am

@@ -36,9 +36,9 @@ DIST_SUBDIRS                            = \
 
 # Always build (e.g. for 'make all') these subdirectories.
 
-SUBDIRS                                 = \
-    posix                                 \
-    $(NULL)
+if OPENTHREAD_EXAMPLES_POSIX
+SUBDIRS                                 = posix
+endif
 
 # Always pretty (e.g. for 'make pretty') these subdirectories.
 

examples/posix/app/Makefile.am

@@ -38,10 +38,16 @@ DIST_SUBDIRS                            = \
 # Always build (e.g. for 'make all') these subdirectories.
 
 SUBDIRS                                 = \
-    cli                                   \
-    ncp                                   \
     $(NULL)
 
+if OPENTHREAD_ENABLE_CLI
+SUBDIRS                                += cli
+endif
+
+if OPENTHREAD_ENABLE_NCP
+SUBDIRS                                += ncp
+endif
+
 # Always pretty (e.g. for 'make pretty') these subdirectories.
 
 PRETTY_SUBDIRS                          = \

examples/posix/app/cli/README.md

@@ -10,7 +10,8 @@ another emulated Thread device.
 
 ```bash
 $ cd openthread
-$ ./bootstrap-configure
+$ ./bootstrap
+$ ./configure --enable-cli --with-examples=posix
 $ make
 ```