Use variables for minimum host versions and bump Python to 3.6

Minimum Git, tar, Python and gcc versions are specified in quite a few
different places. Let's add some variables for these so there's no
chance of missing one if they're updated in future. Additionally, for
hardknott the minimum Python version is 3.6 so set that as the value for
Python.

(From yocto-docs rev: 9a802bc4bb0438c2540f360a08c7787caf64408a)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/faq.rst

@@ -16,7 +16,7 @@ first before being pulled back into Poky. This practice benefits both
 projects immediately.
 
 **Q:** My development system does not meet the required Git, tar, and
-Python versions. In particular, I do not have Python 3.5.0 or greater.
+Python versions. In particular, I do not have Python &MIN_PYTHON_VERSION; or greater.
 Can I still use the Yocto Project?
 
 **A:** You can get the required tools on your host development system a

documentation/ref-manual/system-requirements.rst

@@ -256,11 +256,11 @@ Required Git, tar, Python and gcc Versions
 In order to use the build system, your host development system must meet
 the following version requirements for Git, tar, and Python:
 
--  Git 1.8.3.1 or greater
+-  Git &MIN_GIT_VERSION; or greater
 
--  tar 1.28 or greater
+-  tar &MIN_TAR_VERSION; or greater
 
--  Python 3.5.0 or greater
+-  Python &MIN_PYTHON_VERSION; or greater
 
 If your host development system does not meet all these requirements,
 you can resolve this by installing a ``buildtools`` tarball that
@@ -270,7 +270,7 @@ a pre-built tarball or use BitBake to build the tarball.
 In addition, your host development system must meet the following
 version requirement for gcc:
 
--  gcc 5.0 or greater
+-  gcc &MIN_GCC_VERSION; or greater
 
 If your host development system does not meet this requirement, you can
 resolve this by installing a ``buildtools-extended`` tarball that