[boot] More initrd format changes

CDB seemed to be too simple for the needs of init, and squashfs is too
laden with design choices to work around Linux's APIs. This commit adds
creation of an initrd image of a new format I've called `j6romfs`.

Note that this commit currently does not work! The initrd-reading code
still needs to be added.

scripts/bonnibel/manifest.py

@@ -4,11 +4,6 @@ class Manifest:
     from collections import namedtuple
     Entry = namedtuple("Entry", ("module", "target", "output", "flags"))
 
-    formats = {
-        "none":      0x00,
-        "zstd":      0x01,
-    }
-
     flags = {
         "graphical": 0x01,
         "symbols":   0x80,
@@ -45,11 +40,10 @@ class Manifest:
         self.flags = config.get("flags", tuple())
 
         initrd = config.get("initrd", dict())
-        self.initrd = initrd.get("name", "initrd.dat")
-        self.initrd_format = initrd.get("format", "none")
-
-        if not self.initrd_format in Manifest.formats:
-            raise BonnibelError(f"Manifest specifies unknown initrd format '{self.initrd_format}'")
+        self.initrd = {
+            "name": initrd.get("name", "initrd.dat"),
+            "format": initrd.get("format", "zstd"),
+        }
 
         self.data = []
         for d in config.get("data", tuple()):
@@ -89,15 +83,12 @@ class Manifest:
         with open(path, 'wb') as outfile:
             magic = "jsixboot".encode("utf-8") # magic string
             version = 1
-            reserved = 0
 
-            initrd_format = Manifest.formats.get(self.initrd_format, 0)
             bootflags = sum([Manifest.boot_flags.get(s, 0) for s in self.flags])
 
-            outfile.write(struct.pack("<8s BBH HH",
+            outfile.write(struct.pack("<8s BBH",
                 magic,
-                version, reserved, len(self.panics),
-                initrd_format, bootflags))
+                version, len(self.panics), bootflags))
 
             def write_str(s):
                 data = s.encode("utf-16le")
@@ -118,7 +109,7 @@ class Manifest:
 
             write_ent(self.kernel)
             write_ent(self.init)
-            write_path(self.initrd)
+            write_path(self.initrd["name"])
 
             for p in self.panics:
                 write_ent(p)