Adds support to persist the RTC (if set) across the reboot if entering mass storage mode for on-board flash. The value is then used as the timestamp for the files exposed in the virtual FAT32 filesystem. The files will then have reasonable creation dates when copied to the host computer.
If the RTC is not set (or supported), then the default timestamp of 2018-01-01 will be used (unchanged from previous).
Included some improvements to the RTC functions and exposed the `tz_offsetMinutes` in the `timeConfig` PG. Support already existed for timezone offsets but the parameter wasn't exposed to the user and couldn't be set.
Move timezone offset up a layer as a parameter to systemResetToMsc()
Adds support for specifying a custom timezone offset from both the CLI and MSP calls to enter mass storage mode.
Added an option timezone offset minutes to the CLI `msc` command. If no parameter is specified then the default as specified by `timezone_offset_minutes` will be used. So to reboot into mass storage mode and force the file timestamps to be in UTC, use `msc 0`.
Added reboot message `MSP_REBOOT_MSC_UTC` to support rebooting into mass storage mode and forcing the timestamps to use UTC time (0 offset). The Configurator will need to be modified to use this message for operating systems that expect UTC times for FAT file systems (like Linux).
