[tsmeta.bbclass] fix multiubi build errors due to "override" syntax c…

…hange

In yocto release 3.4(honister) and above the use of "_" to refer to an override is replaced by ":"
Ex. IMAGE_CMD_<some_var> is defined as IMAGE_CMD:<some_var>

References -
https://docs.yoctoproject.org/migration-guides/migration-3.4.html#override-syntax-changes
yoctoproject/poky@32dd3d5

For backward compatibility, kept both formats as some layers still might be using older syntax.

classes/tsmeta.bbclass

@@ -545,7 +545,8 @@ python tsmeta_get_image() {
 
     tempdict = { key.replace("IMAGE_", "").lower(): d.getVar(key) \
         for key in d.keys() if key.startswith("IMAGE_") and \
-        not (key.startswith("IMAGE_CMD_") or key.startswith("IMAGE_FEATURES")) }
+        not (key.startswith("IMAGE_CMD_") or key.startswith("IMAGE_CMD:") \
+        or key.startswith("IMAGE_FEATURES")) }
 
     extra_keys = [
         'EXTRA_IMAGE_INSTALL',