Refactor AES context to be shallow-copyable

Replace RK pointer in AES context with a buffer offset, to allow
shallow copying. Fixes #2147.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/ChangeLog.d/fix-aes-shallow-copying.txt b/ChangeLog.d/fix-aes-shallow-copying.txt
new file mode 100644
index 0000000..0c119d6
--- /dev/null
+++ b/ChangeLog.d/fix-aes-shallow-copying.txt
@@ -0,0 +1,2 @@
+Bugfix
+   * Refactor mbedtls_aes_context to support shallow-copying. Fixes #2147.