ElbeProject: Perform trivial initialisation first

Fixes following error during build:
running cmd +chroot /root/dbprj/example/chroot debconf-set-selections < /root/dbprj/example/chroot/opt/elbe/preseed.txt+
'ElbeProject' object has no attribute '_rpcaptcache

If we exit the constructor early, then self.targetfs, self._rpcaptcache,
self.rpcaptcache_notifier and self.skip_validate were not set. Fix that
by moving trivial assignments to the beginning of the constructor to
prevent this from happening in the future as well.

Signed-off-by: Stefan Gast <stefan.gast@linutronix.de>
Merge remote-tracking branch 'origin/devel/stefan/fixes' into devel/manut