rtc: pass info whether RTC is on UTC as a function parameter

rtc: pass info whether RTC is on UTC as a function parameter

rtc_from_t() and t_from_rtc() call either gmtime or localtime depending
on the value of a global rtc_on_utc variable.

This will not be appropriate anymore when we start exporting functions
that call rtc_from_t() and t_from_rtc() for use outside of rtc_linux.c
as the rtc_on_utc variable may not have been initialized yet or at all.

Therefore make whether the RTC is on UTC a function parameter of these
functions, so the value can be propagated from the callers.