zephyr: Add support for devices without erase and reduced erases

Add Kconfig options:
  - CONFIG_MCUBOOT_STORAGE_WITHOUT_ERASE that enables MCUboot
    configuration MCUBOOT_SUPPORT_DEV_WITHOUT_ERASE
  - CONFIG_MCUBOOT_STORAGE_WITH_ERASE that enables MCUboot
    configuration MCUBOOT_SUPPORT_DEV_WITH_ERASE
  - CONFIG_MCUBOOT_STORAGE_MINIMAL_SCRAMBLE that enables MCUboot
    configuration MCUBOOT_MINIMAL_SCRAMBLE

Adds implementation of flash_area_erase_required, which is required when
MCUBOOT_STORAGE_DEV_WITHOUT_ERASE is enabled.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>