genenv: fix tempdir creation when target is a relative path

when $target is a path relative to $objtree the script fails.
This is because we cd to $basedir and then copy to $tempdir which
then is no longer valid. Fix this by converting $tempdir to an
absolute path first.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Christoph Fritz <chf.fritz@googlemail.com>