[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>

configs/kernel-debug.yaml

@@ -8,6 +8,7 @@ variables:
     "--target=x86_64-unknown-elf",
     "-I${source_root}/external",
 
+    "-nostdinc",
     "-nostdlib",
     "-ffreestanding",
     "-nodefaultlibs",
@@ -18,7 +19,8 @@ variables:
     "-mno-red-zone",
     "-mcmodel=large",
 
-    "-g",
+    "-g3",
+    "-ggdb",
 
     "-D__ELF__",
     "-D__JSIX__",