boot: zephyr: Check for unexpected flash sector size not compatible w/ series with variable sector sizes

[erwango]

#2059 introduce sanity check based on optional property erase-block-size.

This prevents use of mcuboot on platform not defining this property.
One example are STM32F2, STM32F4 and STM32F7 series which have variable sector size. On these series, erase-block-size depends on sector size and then cannot be defined as done usually in flash controller parent.

Check should have a fallback for this case.