Support virtual dev-switch (keyboard-based dev-mode)

BUG=chrome-os-partner:9706
TEST=manual

Currently, Link is the only platform that enables this feature.

To enter dev-mode:

  Boot into recovery mode using the magic key chord. At the Insert screen,
  press Ctrl-D. You'll be asked if you want to enter developer mode. If you
  then press ENTER, it will reboot with dev-mode enabled. If you press SPACE
  or ESC, it will return to the Insert screen.

  If you enter recovery mode through any other means, or if dev-mode is
  already enabled, pressing Ctrl-D at the Insert screen will have no effect.

To return to normal mode:

  Reboot. At the Dev screen, press ENTER or SPACE. It will reboot to
  recovery mode and ask you if you want to return to normal mode. If you
  press ESC or power off, you'll still be in dev-mode. Press ENTER or SPACE,
  and it will reboot into normal mode (of course, if you've messed up your
  images while in dev-mode, you'll just come right back to recovery mode
  again).

  You can also request a direct return to normal mode by running

    crossystem disable_dev_request=1

  and rebooting.

Change-Id: I435905855a6c39932ee466cc046bdc4c4c860f98
Reviewed-on: https://gerrit.chromium.org/gerrit/24160
Tested-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Commit-Ready: Bill Richardson <wfrichar@chromium.org>

firmware/include/vboot_struct.h

@@ -231,6 +231,8 @@ typedef struct VbKernelPreambleHeader {
 #define VBSD_BOOT_S3_RESUME             0x00000100
 /* Read-only firmware supports the normal/developer code path */
 #define VBSD_BOOT_RO_NORMAL_SUPPORT     0x00000200
+/* VbInit was told that the system has a virtual dev-switch */
+#define VBSD_HONOR_VIRT_DEV_SWITCH      0x00000400
 
 /* Result codes for VbSharedDataHeader.check_fw_a_result (and b_result) */
 #define VBSD_LF_CHECK_NOT_DONE          0