[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/boot/loader.h

@@ -11,34 +11,30 @@ namespace bootproto {
 
 namespace boot {
 
+class descriptor;
+
 namespace fs {
     class file;
 }
 
 namespace loader {
 
-struct program_desc
-{
-    const wchar_t *name;
-    const wchar_t *path;
-};
-
 /// Load a file from disk into memory.
 /// \arg disk  The opened UEFI filesystem to load from
 /// \arg desc  The program descriptor identifying the file
 util::buffer
 load_file(
     fs::file &disk,
-    const program_desc &desc);
+    const descriptor &desc);
 
 /// Parse and load an ELF file in memory into a loaded image.
 /// \arg disk       The opened UEFI filesystem to load from
-/// \arg desc       The program descriptor identifying the program
+/// \arg desc       The descriptor identifying the program
 /// \arg add_module Also create a module for this loaded program
 bootproto::program *
 load_program(
     fs::file &disk,
-    const program_desc &desc,
+    const descriptor &desc,
     bool add_module = false);
 
 /// Load a file from disk into memory, creating an init args module
@@ -47,7 +43,7 @@ load_program(
 void
 load_module(
     fs::file &disk,
-    const program_desc &desc);
+    const descriptor &desc);
 
 /// Verify that a loaded ELF has the j6 kernel header
 /// \arg program  The program to check for a header