There seems to be a bug in the bootrom for the rp2040 which means
that the chip erase command is not accepted. This is because the
CS pin must be released (set high) directly after sending the chip
erase command (0x60 or 0xC7) (see Winbond W25Q128JV datasheet for
details). Instead the bootrom sends the address after the command,
thus the SPI flash silently ignores the command. Instead, we must
erase each 64KB block one at a time, but thankfull the bootrom
handles this correctly for us.
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