BL2: Sanity check value in x0 in the entry point code

Change-Id: Icef68e314e6ba0f3694189b57f4b1dbbea5ba255

bl2/aarch64/bl2_entrypoint.S

@@ -77,6 +77,14 @@ bl2_entrypoint:; .type bl2_entrypoint, %function
 
 	isb
 
+	/* ---------------------------------------------
+	 * Check the opcodes out of paranoia.
+	 * ---------------------------------------------
+	 */
+	mov	x0, #RUN_IMAGE
+	cmp	x0, x20
+	b.ne	_panic
+
 	/* ---------------------------------------------
 	 * Zero out NOBITS sections. There are 2 of them:
 	 *   - the .bss section;