[init] Add new initrd format

A new compressed initrd format for srv.init to load drivers, services,
and data from, instead of every file getting loaded by the bootloader.
This will allow for less memory allocated by the bootloader and passed
to init if not every driver or data file is loaded.

Loading, passing, and using the new initrd will be done in a coming
commit.

configs/rules.ninja

@@ -66,6 +66,10 @@ rule makefat
         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 = $