implement time syscalls
This change adds optional support for libc time functions.
New menuconfig option allows selecting FRC1, RTC, both, or none to be used for timekeeping.
- If only FRC1 timer is used, gettimeofday will provide time at
microsecond resolution. Time will not be preserved when going
into deep sleep mode.
- If both FRC1 and RTC timers are used, timekeeping will
continue in deep sleep. Time will be reported at 1 microsecond
resolution.
- If only RTC timer is used, timekeeping will continue in
deep sleep, but time will be measured at 6.(6) microsecond
resolution. Also the gettimeofday function itself may take
longer to run.
- If no timers are used, gettimeofday function will return -1 and
set errno to ENOSYS.
`times` function returns time derived from FreeRTOS ticks. It reports all as 'system' time, 'user' time is reported as zero.
`settimeofday` function may be used to set current time.
LwIP SNTP module is hooked up into the system through `settimeofday`/`gettimeofday`.
Example demonstrating this functionality is also added.
ref. TW6415
See merge request !168
abecab7525