New firmware release v1.17.3.b1

This release is mainly here to fix some problems introduced by 1.17.2.b1 a few days ago.
Here's the change log:

• esp32: Switch off the heartbeat LED right after boot.
• esp32/ftp/updater.c: updated function updater_write_boot_info() to write encrypted header (if flash encryption enabled)
• esp32/ftp/updater.c: removed code which checked in ota_write() if chunks are multiple of 16 bytes
• esp32: Use esp_timer_get_time instead of gettimeofday inside ISRs and the idle task (as recommended by Espressif). Fixes crashes caused when using time.ticks_ms() and ticks_us()
• esp32: Only create the LTE task once. Fixes soft reset for the FiPy and GPy.
• esp32: Correct the heartbeat behaviour during soft reset.

Please note that that the function ticks_cpu (https://docs.pycom.io/chapter/firmwareapi/micropython/utime.html#utimetickscpu) has been changed and it now returns a value in microseconds (it's useless to return nano-seconds anyway), so it can be seen as another version of ticks_us but a lot faster. The nano-gateway example has been updated to use this function instead: pycom/pycom-libraries@f344b5a