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 rev: 073610af04be326f9245ca91714526b390fb72cd)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/rootfs-postcommands.bbclass

@@ -307,6 +307,7 @@ python write_image_test_data() {
            os.remove(testdata_link)
         os.symlink(os.path.basename(testdata), testdata_link)
 }
+write_image_test_data[vardepsexclude] += "TOPDIR"
 
 # Check for unsatisfied recommendations (RRECOMMENDS)
 python rootfs_log_check_recommends() {