[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.

src/boot/bootconfig.h

@@ -31,12 +31,10 @@ public:
     inline const descriptor & kernel() const { return m_kernel; }
     inline const descriptor & init() const { return m_init; }
     inline const wchar_t * initrd() const { return m_initrd; }
-    inline uint16_t initrd_format() const { return m_initrd_format; }
     inline const descriptors & panics() { return m_panics; }
 
 private:
     uint16_t m_flags;
-    uint16_t m_initrd_format;
     descriptor m_kernel;
     descriptor m_init;
     descriptors m_panics;