generate-manifest-2.7.py: add logic to generate native manifest

generate-manifest-2.7.py: add logic to generate native manifest

python-native supposes to RPROVIDE all native packages as added in
generate-manifest-2.7.py, but it does not so far, this leads a problem
that sometimes bitbake cant find a runtime provider for a python-*-native
when a new runtime dependency on it being required, this usualy happens
after a new native python-* recipe is created or the old native python-*
recipes are upgraded.

To give a example, the following commit is trying to address such a issue:
commit 4583cd1bb15306e8f0ab7bcd80732e6f35aa4533:
[ python-native: Make python-native also RPROVIDE python-unittest-native ]

To avoid manually extending RPROVIDE every time when a new runtime
dependency is introduced, an argument '-n/--native' is added to the
manifest generator, allowing it create a native python manifest, with a
RPROVIDE line only, the RPROVIDE should contain all the sub-packages.

The generated python-native-2.7-manifest.inc is also added which is
included by python-native recipe.

(From OE-Core rev: 0cb15d9559e34faffea1ac0be825d0602f225ba9)

Signed-off-by: Ming Liu <peter.x.liu@external.atlascopco.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>