[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.cpp

@@ -0,0 +1,62 @@
+#include <uefi/boot_services.h>
+#include <uefi/types.h>
+
+#include "bootconfig.h"
+#include "error.h"
+#include "fs.h"
+#include "status.h"
+
+namespace boot {
+
+constexpr uint64_t jsixboot = 0x746f6f627869736a; // "jsixboot"
+
+static const wchar_t *
+read_string(util::buffer &data)
+{
+    uint16_t size = *util::read<uint16_t>(data);
+    const wchar_t *string = reinterpret_cast<const wchar_t*>(data.pointer);
+    data += size;
+    return string;
+}
+
+static void
+read_descriptor(descriptor &e, util::buffer &data)
+{
+    e.flags = static_cast<desc_flags>(*util::read<uint16_t>(data));
+    e.path = read_string(data);
+    e.desc = read_string(data);
+}
+
+bootconfig::bootconfig(util::buffer data, uefi::boot_services *bs)
+{
+    status_line status {L"Loading boot config"};
+
+    if (*util::read<uint64_t>(data) != jsixboot)
+        error::raise(uefi::status::load_error, L"Bad header in jsix_boot.dat");
+
+    const uint8_t version = *util::read<uint8_t>(data);
+    if (version != 0)
+        error::raise(uefi::status::incompatible_version, L"Bad version in jsix_boot.dat");
+
+    data += 1; // reserved byte
+    uint16_t num_programs = *util::read<uint16_t>(data);
+    uint16_t num_data = *util::read<uint16_t>(data);
+    data += 2; // reserved short
+
+    read_descriptor(m_kernel, data);
+    read_descriptor(m_init, data);
+
+    m_programs.count = num_programs;
+    m_programs.pointer = new descriptor [num_programs];
+
+    for (unsigned i = 0; i < num_programs; ++i)
+        read_descriptor(m_programs[i], data);
+
+    m_data.count = num_programs;
+    m_data.pointer = new descriptor [num_data];
+
+    for (unsigned i = 0; i < num_data; ++i)
+        read_descriptor(m_data[i], data);
+}
+
+} // namespace boot