[boot] Create bootconfig to tell boot what to load

While bonnibel already had the concept of a manifest, which controls
what goes into the built disk image, the bootloader still had filenames
hard-coded. Now bonnibel creates a 'jsix_boot.dat' file that tells the
bootloader what it should load.

Changes include:

- Modules have two new fields: location and description. location is
  their intended directory on the EFI boot volume. description is
  self-explanatory, and is used in log messages.
- New class, boot::bootconfig, implements reading of jsix_boot.dat
- New header, bootproto/bootconfig.h, specifies flags used in the
  manifest and jsix_boot.dat
- New python module, bonnibel/manifest.py, encapsulates reading of the
  manifest and writing jsix_boot.dat
- Syntax of the manifest changed slightly, including adding flags
- Boot and Kernel target ccflags unified a bit (this was partly due to
  trying to get enum_bitfields to work in boot)
- util::counted gained operator+= and new free function util::read<T>

src/libraries/util/include/util/counted.h

@@ -35,6 +35,14 @@ struct counted
 
     /// Return an iterator to the end of the array
     inline const_iterator end() const { return offset_pointer<const T>(pointer, sizeof(T)*count); }
+
+    /// Advance the pointer by N items
+    inline counted<T> & operator+=(unsigned i) {
+        if (i > count) i = count;
+        pointer += i;
+        count -= i;
+        return *this;
+    }
 };
 
 /// Specialize for `void` which cannot be indexed or iterated
@@ -43,8 +51,24 @@ struct counted<void>
 {
     void *pointer;
     size_t count;
+
+    /// Advance the pointer by N bytes
+    inline counted<void> & operator+=(unsigned i) {
+        if (i > count) i = count;
+        pointer = offset_pointer(pointer, i);
+        count -= i;
+        return *this;
+    }
 };
 
 using buffer = counted<void>;
 
+template <typename T>
+const T * read(buffer &b) {
+    const T *p = reinterpret_cast<const T*>(b.pointer);
+    b.pointer = offset_pointer(b.pointer, sizeof(T));
+    b.count -= sizeof(T);
+    return p;
+}
+
 } // namespace util