If the SoftDevice is enabled: This call initiates the flash access
command, and its completion will be communicated to the application
with exactly one of the following events:
NRF_EVT_FLASH_OPERATION_SUCCESS - The command was successfully
completed. NRF_EVT_FLASH_OPERATION_ERROR - The command could not be
started.
If the SoftDevice is not enabled no event will be generated,
and this call will return NRF_SUCCESS when the write has been
completed
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It is actually documented here https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s132.api.v3.0.0/group___n_r_f___s_o_c___f_u_n_c_t_i_o_n_s.html?cp=5_7_3_9_2_7_2_9#ga8b49f2e72e97291aecc18ce396956eed
so I think registering jsh_sys_evt_handler above when BLUETOOTH is not defined is actually not needed, all it does now is here
https://github.com/espruino/Espruino/blob/52352efac4f8f40976a4d268a0a074ad15251b29/targets/nrf5x/jshardware.c#L638