devtool/upgrade: raise an error if extracting source produces more than one directory

devtool/upgrade: raise an error if extracting source produces more than one directory

This can happen if running unpack task produces unrelated files and directories
(e.g. if recipe_qa or other tasks run); in this case it's better to stop, rather
than allow devtool to continue and error out much later with a message that has
nothing to do with where the problem originated.

The idea here was to handle tarballs that don't contain a top level directory and thus
the source tree is one level up; this basically never happens, and if it does we
should find a less brittle way to handle such tarballs.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>