[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/kernel/kernel.module

@@ -5,6 +5,7 @@ kernel = module("kernel",
     default = True,
     output = "jsix.elf",
     targets = [ "kernel" ],
+    description = "jsix kernel",
     deps = [ "util", "cpu", "bootproto" ],
     includes = [ "." ],
     sources = [