updatepkg: Get version number from current XML

...instead of the base XML file, as the current source XML changes when
we install new packages or change the version number.

Signed-off-by: Stefan Gast <stefan.gast@linutronix.de>
Signed-off-by: Manuel Traut <manut@linutronix.de>

Conflicts:
	elbepack/updatepkg.py
docs: Fix target/xmlfile confusion in gen_update

The given XML file is from the base version of the project. The
currently installed projects are compared against the fullpkg list in
the XML file. The update package will upgrade a project created with the
given XML file to the packages currently installed in the given target
directory.

Signed-off-by: Stefan Gast <stefan.gast@linutronix.de>
Signed-off-by: Manuel Traut <manut@linutronix.de>

Conflicts:
	docs/elbe-gen_update.txt