There is no difference between 4k erase and sector erase.
But sector erase would previously do `.block_erase` which would not
erase a sector (4k) but a whole block (64k).
Fortunately all boards declare `SPI_NOR_F_SECT_4K` and all file systems
don't try to erase anything smaller, so this was never triggered.
Add an `assert(0)` to crash instead of corrupting data.
The pad for SPI-Flash on the WeAct-f411ce does not come pre-populated.
The user has to manually solder a SPI-flash chip on the board, so specifying
a size here makes no sense.
Instead, rely on automatic detection of the flash size at run-time.
