Ramdisk in the manifest, simplify initrd generation

We currently support loading a ramdisk for the primary. In fact, all
primaries are required to have one called 'initrd.img'. This makes
the ramdisk optional and specifies its filename in the manifest.

With that, generate_initrd.py loses the last remaining hardcoded
assumption about the structure of the Hafnium ramdisk. Refactor the
script and the build so that generate_initrd.py simply accepts a list of
(name in ramdisk, path on host) pairs.

Bug: 117551352
Change-Id: Iff2d70205940a1740c473d1cac34d5a05d6d6791
diff --git a/inc/hf/manifest.h b/inc/hf/manifest.h
index 23689d9..3e9f76f 100644
--- a/inc/hf/manifest.h
+++ b/inc/hf/manifest.h
@@ -29,11 +29,17 @@
 	struct string debug_name;
 	struct string kernel_filename;
 
-	/* Properties specific to secondary VMs. */
-	struct {
-		uint64_t mem_size;
-		spci_vcpu_count_t vcpu_count;
-	} secondary;
+	union {
+		/* Properties specific to the primary VM. */
+		struct {
+			struct string ramdisk_filename;
+		} primary;
+		/* Properties specific to secondary VMs. */
+		struct {
+			uint64_t mem_size;
+			spci_vcpu_count_t vcpu_count;
+		} secondary;
+	};
 };
 
 /**