remove hwclock -r test and add /dev/rtc0 exists test

[joshschmelzle]

Problem with hwclock in during assembly:

# hwclock -vv
hwclock from util-linux 2.36.1
System Time: 1641002098.618969
Trying to open: /dev/rtc0
Using the rtc interface to the clock.
Assuming hardware clock is kept in UTC time.
Waiting for clock tick...
ioctl(4, RTC_UIE_ON, 0): Invalid argument
Waiting in loop for time from /dev/rtc0 to change
hwclock: ioctl(RTC_RD_TIME) to /dev/rtc0 to read the time failed: Invalid argument
...synchronization failed

PR removes the hwclock -r test which is likely getting a non-zero return value.

PR adds an existence test for /dev/rtc0:

wlanpi@wlanpi:~ $ ls /dev/rtc0
/dev/rtc0
wlanpi@wlanpi:~ $ echo $?
0