usb_update: add extra command "UPDATE_EXTRA_CMD_CONSOLE_READ_*"

Similar to corressponding host commands, we can read uart console buffer
by following ways:

A. Read from the beginning of buffer to the end of buffer:
  # 1. set snapshot before reading
  UPDATE_EXTRA_CMD_CONSOLE_READ_INIT
  while (true) {
    # 2. read 64 bytes back
    UPDATE_EXTRA_CMD_CONSOLE_READ_NEXT CONSOLE_READ_NEXT
    # 3. if (2) returns an empty string, break, otherwise, continue.
  }

B. Mimic `dmesg -w` (keep reading new messages)
  while (true) {
    # 1. set snapshot before reading
    UPDATE_EXTRA_CMD_CONSOLE_READ_INIT
    while (true) {
      # 2. read 64 bytes back
      UPDATE_EXTRA_CMD_CONSOLE_READ_NEXT CONSOLE_READ_RECENT
      # 3. if (2) returns an empty string, break, otherwise, continue.
    }
  }

Add argument `-l` to usb_updater2, which will perform (B).  Note that
the update interface will be occupied while `usb_updater2 -l` is still
running, so you can't use other updater command at the same time.

BRANCH=none
BUG=b:112877237
TEST=test on whiskers
Signed-off-by: Wei-Han Chen <stimim@chromium.org>

Change-Id: I8d2010f84602ca6b84034a0cabe42ae7441614e0
Reviewed-on: https://chromium-review.googlesource.com/1177293
Commit-Ready: Wei-Han Chen <stimim@chromium.org>
Tested-by: Wei-Han Chen <stimim@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/board/hammer/board.h b/board/hammer/board.h
index 9e48463a98..d797e198cd 100644
--- a/board/hammer/board.h
+++ b/board/hammer/board.h
@@ -282,6 +282,8 @@
 
 #define CONFIG_USB_PAIRING
 
+#define CONFIG_USB_CONSOLE_READ
+
 #ifdef BOARD_WAND
 /* Battery and charger options. */
 #define CONFIG_CHARGER