classes: drop image dependencies on TOPDIR variable
We don't need a dependency on this variable changing, and having one
causes locked signature warnings during eSDK installation if you have
INITRAMFS_IMAGE_* set (since TOPDIR will always be different between
the eSDK and the environment in which it was built).
Relates to [YOCTO #12102].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
