watchdog: Fix Watchdog Reset while in U-Boot Prompt

Hardware: CM-FX6 Module from Compulab

This patch fixes unwanted watchdog resets while the user enters
a command at the U-Boot prompt.

As found on the CM-FX6 board from Compulab, when having enabled the
watchdog, a missing WATCHDOG_RESET call in common/console.c causes
this and alike boards to reset when the watchdog's timeout has
elapsed while waiting at the U-Boot prompt.

Despite the user could press several keys within the watchdog
timeout limit, the while loop in cli_readline.c, line 261, does only
call WATCHDOG_RESET if first == 1, which gets set to 0 in the 1st
loop iteration. This leads to a watchdog timeout no matter if the
user presses keys or not.

Although, this affects other boards as well as it touches
common/console.c, the macro WATCHDOG_RESET expands to {} if watchdog
support isn't configured. Hence, there's no harm caused and no need to
surround it by #ifdef in this case.

 * Symptom:
   U-Boot resets after watchdog times out when in commandline prompt
   and watchdog is enabled.

 * Reasoning:
   When U-Boot shows the commandline prompt, the following function
   call stack is executed while waiting for a keypress:

   common/main.c:
                    main_loop          => common/cli.c: cli_loop() =>
   common/cli_hush.c:
                    parse_file_outer   => parse_stream_outer       =>
                    parse_stream       => b_getch(i)               =>
                    i->get(i)          => file_get                 =>
                    get_user_input     => cmdedit_read_input       =>
                    uboot_cli_readline =>
   common/cli_readline.c:
                    cli_readline       => cli_readline_into_buffer =>
                    cread_line         => getcmd_getch (== getc)   =>
   common/console.c:
                    fgetc              => console_tstc

   common/console.c:
   (with CONFIG_CONSOLE_MUX is set)

   - in console_tstc line 181:
   If dev->tstc(dev) returns 0, the global tstcdev variable doesn't get
   set. This is the case if no character is in the serial buffer.

   - in fgetc(int file), line 297:
   Program flow keeps looping because tstcdev does not get set.
   Therefore WATCHDOG_RESET is not called, as mx_serial_tstc from
   drivers/serial/serial_mxc.c does not call it.

 * Solution:
   Add WATCHDOG_RESET into the loop of console_tstc.

   Note: Macro expands to {} if not configured, so no #ifdef is needed.

 * Comment:

Signed-off-by: Christian Storm <christian.storm@tngtech.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andreas J. Reichel <Andreas.Reichel@tngtech.com>
Acked-by: Simon Glass <sjg@chromium.org>