Convert extension version assignment to be simple

To match what was done in I4798fb69c639e6d52d275a5b793fa0c8ba876202.

Test: m nothing
Merged-In: I4798fb69c639e6d52d275a5b793fa0c8ba876202
Change-Id: I65b7c5974e0ce64bb43474e49ef9ed8d2b13d001
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index d65e697..88fa427 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -156,7 +156,8 @@
 .KATI_READONLY := PLATFORM_SDK_VERSION
 
 # This is the sdk extension version of this tree.
-PLATFORM_SDK_EXTENSION_VERSION :=$= 1
+PLATFORM_SDK_EXTENSION_VERSION := 1
+.KATI_READONLY := PLATFORM_SDK_EXTENSION_VERSION
 
 # This is the sdk extension version that PLATFORM_SDK_VERSION ships with.
 PLATFORM_BASE_SDK_EXTENSION_VERSION :=$= 0