[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.

configs/rules.ninja

@@ -60,16 +60,16 @@ rule makest
     description = Making symbol table
     command = nm -n -S --demangle $in | ${source_root}/scripts/build_symbol_table.py $out
 
+rule makeinitrd
+    description = Creating $name
+    command = ${source_root}/scripts/mkj6romfs.py -c $format $in $out
+
 rule makefat
     description = Creating $name
     command = $
         cp $in $out; $
         mcopy -s -D o -i $out@@1M ${build_root}/fatroot/* ::/
 
-rule mkinitrd
-    description = Creating initrd
-    command = ${source_root}/scripts/mkinitrd.py $out $in
-
 rule strip
     description = Stripping $name
     command = $