Treat missing compression node in FIT image as IH_COMP_NONE.
This is implicentely already happening in most places, but for now
was still triggering an annoying warning about initramfs compression
being obsolete despite compression note being absent.
Fix this.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 0a18456ffc)
