Some watchdog drivers implement WDIOF_CARDRESET feature. As example,
see commit b6ef36d2 ("watchdog: qcom: Report reboot reason").
This option allows reporting to userspace the cause of the last boot
(POR/watchdog reset), being helpful in e.g. automated test-cases.

Add support for WDIOC_GETBOOTSTATUS in the test code, to be able to:
- check if watchdog drivers properly implement WDIOF_CARDRESET.
- check the last boot status, if WDIOF_CARDRESET is implemented.

Make the `-b, --bootstatus` option one-shot. That means, skip the
keepalive mechanism if `-b` is provided on the command line, as we
are only interested in the boot status information.

Tested on Rcar-H3 Salvator-X board:

********************** Cold boot finished
salvator-x:/home/root# ./watchdog-test -h
Usage: ./watchdog-test [options]
 -b, --bootstatus    Get last boot status (Watchdog/POR)
 -d, --disable       Turn off the watchdog timer
 -e, --enable        Turn on the watchdog timer
 -h, --help          Print the help message
 -p, --pingrate=P    Set ping rate to P seconds (default 1)
 -t, --timeout=T     Set timeout to T seconds

Parameters are parsed left-to-right in real-time.
Example: ./watchdog-test -d -t 10 -p 5 -e
salvator-x:/home/root#
salvator-x:/home/root# ./watchdog-test -b
Last boot is caused by: Power-On-Reset.
salvator-x:/home/root#
salvator-x:/home/root# ./watchdog-test -d -t 1 -p 2 -e
Watchdog card disabled.
Watchdog timeout set to 1 seconds.
Watchdog ping rate set to 2 seconds.
Watchdog card enabled.
Watchdog Ticking Away!
********************** Reboot due to watchdog trigger finished
salvator-x:/home/root# ./watchdog-test -b
Last boot is caused by: Watchdog.
salvator-x:/home/root#
salvator-x:/home/root# reboot
********************** Reboot due to user action finished
salvator-x:/home/root# ./watchdog-test -b
Last boot is caused by: Power-On-Reset.
salvator-x:/home/root#
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>