fs: littlefs: sample: Switch to FIXED_PARTITION_ macros
fs: littlefs: sample: Switch to FIXED_PARTITION_ macros

The commit switches flash area access from FLASH_AREA_ macros
to FIXED_PARTITION_ macros and to usage of DTS node labels,
to identify partitions, instead of label property.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr