fs: fcb: Make FCB work with sectors larger than 16K

Enhance FCB to also work with sectors larger than 16K and
to handle larger flash alignment constraints correctly.

use fcb_len_in_flash when setting the offset of the data
and use buffers sizes of at least the alignment value.

The test in fcb_test_append_to_big has been altered, as it
would otherwise not come to a data length which fits the
fcb on sectors larger than 16K.

Closes: https://github.com/zephyrproject-rtos/zephyr/issues/45345



Signed-off-by: Achatzi Julian <jachatzi@baumer.com>