[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/bootconfig.h

@@ -0,0 +1,43 @@
+/// \file bootconfig.h
+/// Definitions for reading the jsix bootconfig file
+#pragma once
+
+#include <bootproto/bootconfig.h>
+#include <util/counted.h>
+
+namespace uefi {
+    struct boot_services;
+}
+
+namespace boot {
+
+using desc_flags = bootproto::desc_flags;
+
+struct descriptor {
+    desc_flags flags;
+    wchar_t const *path;
+    wchar_t const *desc;
+};
+
+/// A bootconfig is a manifest of potential files.
+class bootconfig
+{
+public:
+    using descriptors = util::counted<descriptor>;
+
+    /// Constructor. Loads bootconfig from the given buffer.
+    bootconfig(util::buffer data, uefi::boot_services *bs);
+
+    inline const descriptor & kernel() { return m_kernel; }
+    inline const descriptor & init() { return m_init; }
+    descriptors programs() { return m_programs; }
+    descriptors data() { return m_data; }
+
+private:
+    descriptor m_kernel;
+    descriptor m_init;
+    descriptors m_programs;
+    descriptors m_data;
+};
+
+} // namespace boot