debug: coredump: flash backend: fix header if stream erase

If CONFIG_STREAM_FLASH_ERASE is set, a page erase is done before
writing the coredump header to the flash. If the flash page erase size
is larger than the flash write size this results in erasing part of
the coredump data.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>