Support alternative GPT header signature

In order to dual boot Windows and ChromeOS, Windows must
not find a GPT partition table on the disk. So change
ChromeOS to cope with an alternative signature "CHROMEOS"
instead of the standard "EFI PART"

BUG=chrome-os-partner:6108
TEST=rebuild chromeos, install it,
     run cgpt legacy /dev/sda
     dd if=/dev/sda of=/tmp/x bs=1k
     hexdump -C /tmp/X
     see the string CHROMEOS
BRANCH=link
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>

Change-Id: Ia88eff33b9880bd73a78c1b8e026c1f8298c4557
Reviewed-on: https://gerrit.chromium.org/gerrit/31264
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Ready: Stefan Reinauer <reinauer@chromium.org>
Tested-by: Stefan Reinauer <reinauer@chromium.org>
diff --git a/cgpt/cgpt.h b/cgpt/cgpt.h
index 6851ded..b3f7ff3 100644
--- a/cgpt/cgpt.h
+++ b/cgpt/cgpt.h
@@ -154,6 +154,7 @@
 int cmd_boot(int argc, char *argv[]);
 int cmd_find(int argc, char *argv[]);
 int cmd_prioritize(int argc, char *argv[]);
+int cmd_legacy(int argc, char *argv[]);
 
 #define ARRAY_COUNT(array) (sizeof(array)/sizeof((array)[0]))
 const char *GptError(int errnum);