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].
(From OE-Core master rev:
073610af04be326f9245ca91714526b390fb72cd)
(From OE-Core rev:
94a9f9a7dc5a736b7986b889895c736f02110715)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
