nrf: fix internal flash writes

[dhalbert]

• Logic to wait for softdevice flash operation events was wrong.
• Trying to write a full 4096-byte flash page causes BLE timeouts in the softdevice. This was discovered several months ago by @hathach while doing bootloader work. See https://devzone.nordicsemi.com/f/nordic-q-a/40088/sd_flash_write-cause-nrf_fault_id_sd_assert/. To fix, we write only half a page at a time.
• Reads from flash did not flush the write cache first.

Still thinking a bit about when else to flush the cache, maybe using auto-reload timer (even if we are not running a code.py). PR for now is for testing -- don't merge quite yet.

Fixes #1654.

[dhalbert]

@jerryneedell @uhreber This PR has an additional filesystem flush beyond the .uf2's I gave you. @jerryneedell, this may fix the "missing file" you mentioned at the end of this comment.

[jerryneedell]

downloaded and tested on PCA10059 -- ran same sequence as before:
erased FS
reloaded libs
reloaded ble_uart_echo_test
this time all files were present

ran ble_uart_echo_test
copied 5Kbyte file to CIRCUITPY a few times
no issues

[jerryneedell]

tested on PCA10059 - unable to reproduce any of the FS issues I reported previously.