README: Update pylibfdt install instructions

Now that pip is supported for installs, update the install instructions to
use it. Using pip over setup.py is generally recommended and simpler.

Also, drop 'SETUP_PREFIX' as it doesn't exist anywhere.

Signed-off-by: Rob Herring <robh@kernel.org>
Message-Id: <20211112041633.741598-2-robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

README

@@ -48,18 +48,24 @@ If you add new features, please check code coverage:
     # Open 'htmlcov/index.html' in your browser
 
 
-To install the library via the normal setup.py method, use:
+The library can be installed with pip from a local source tree:
 
-    ./pylibfdt/setup.py install [--prefix=/path/to/install_dir]
+    pip install . [--user|--prefix=/path/to/install_dir]
 
-If --prefix is not provided, the default prefix is used, typically '/usr'
-or '/usr/local'. See Python's distutils documentation for details. You can
-also install via the Makefile if you like, but the above is more common.
+Or directly from a remote git repo:
+
+    pip install git+git://git.kernel.org/pub/scm/utils/dtc/dtc.git@main
+
+The install depends on libfdt shared library being installed on the host system
+first. Generally, using --user or --prefix is not necessary and pip will use the
+default location for the Python installation which varies if the user is root or
+not.
+
+You can also install everything via make if you like, but pip is recommended.
 
 To install both libfdt and pylibfdt you can use:
 
-    make install [SETUP_PREFIX=/path/to/install_dir] \
-            [PREFIX=/path/to/install_dir]
+    make install [PREFIX=/path/to/install_dir]
 
 To disable building the python library, even if swig and Python are available,
 use: