boards: riscv: tlsr9518adk80d: add IMG_BLOCK_BUF_SIZE to board defconfig

Add IMG_BLOCK_BUF_SZE config into tlsr9518adk80d_defconfig due to
limitations for buffer image writter. The buffer shall be less (multiple
of access alignment) or equal to flash page. tlsr9518adk80d boards use
external P25Q16 IC as flesh memory. Flash page size of the IC is 256
bytes. So that, it is maximum image writer buffer size for such kind of
boards.

Signed-off-by: Alexandr Kolosov <rikorsev@gmail.com>