fs: littlefs: Extend sample program to test block devices

Up till now, the littlefs sample program was only supporting the flash
medium. However, it would be also welcome to be able to test reading and
writing data from littlefs stored on block devices - like SD card.

In this case we do use the block API, to read and write data to it.
To achieve this goal - the CONFIG_APP_LITTLEFS_STORAGE_BLK_SDMMC define
has been introduced.

One can change the disk mount point by defining board specific
CONFIG_SDMMC_VOLUME_NAME.

The test for nucleo board equipped with SD card shall be built with:
west build -p always -b nucleo_h743zi ./zephyr/samples/subsys/fs/littlefs \
 -DCONF_FILE=prj_blk.conf

Moreover, the README.rst has been extended to describe running this
test on block devices.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
6 files changed