boards: riscv: tlsr9518adk80d: Move IMG_BLOCK_BUF_SIZE in Kconfig.defconfig

While build of samples or applications which does not contain
MCUBOOT_IMG_MANAGER option enabled, following warning occures:

warning: IMG_BLOCK_BUF_SIZE was assigned the value 256 but got
the value ``.

Because of that it decided to move IMG_BLOCK_BUF_SIZE config to
Kconfig.defconfig with additional condition if MCUBOOT_IMG_MANAGER

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