This fixes the EFI BIOS boot problems introduced with cgpt.

There were two changes. First, we need to pack the GPT header to make it
match the spec (duh). Second, there's a subtle bug in how the BIOS recovers
from corrupted headers. The EFI spec says that the primary GPT header must
be at sector 1 (counting from zero) and the secondary GPT header must be at
the last sector on the drive. The BIOS correctly looks in those locations to
find the headers. However, if the secondary GPT header is invalid (as it
usually is due to our build process), the BIOS is supposed to update it from
the primary header. In this case, rather than write to the last sector on
the drive where it just looked, the BIOS trusts the alternate_lba field of
the primary header. That field is supposed to point to the secondary header
location, but the BIOS just blindly uses it no matter where it points. The
cgpt tool wasn't initializing that field, so it pointed to sector 0, which
is the PMBR. The BIOS overwrote that, resulting in an unbootable drive.

Review URL: http://codereview.chromium.org/2844006

vboot_firmware/lib/cgptlib/include/gpt.h

@@ -79,13 +79,12 @@ typedef struct {
   uint64_t first_usable_lba;
   uint64_t last_usable_lba;
   Guid disk_uuid;
-
   uint64_t entries_lba;
   uint32_t number_of_entries;
   uint32_t size_of_entry;
   uint32_t entries_crc32;
   uint8_t reserved_padding[];           /* entire sector reserved for header */
-} GptHeader;
+} __attribute__((packed)) GptHeader;
 
 /* GPT partition entry defines the starting and ending LBAs of a partition.
  * It also contains the unique GUID, type, and attribute bits.
@@ -100,6 +99,6 @@ typedef struct {
   uint64_t attributes;
   uint16_t name[36];                    /* UTF-16 encoded partition name */
   uint8_t reserved[];                   /* nothing, really */
-} GptEntry;
+} __attribute__((packed)) GptEntry;
 
 #endif  /* VBOOT_REFERENCE_CGPTLIB_GPT_H_ */