subsys: fs/nvs: Improved nvs for larger blocksizes

The nvs module has some disadvantages for larger block size. The data
header and slot are taking up to much space. A rewrite is proposed that
reduces the used storage space for systems with write block size > 4.

The data storage in flash is now one unit consisting of: data_length,
data_id, data and data_length again in a multiple of the write block
size. The data_length at the end is used to validate the correctness of
the flash write and also allows to travel backwards in the filesystem.

As a comparison, on a system with block size 8 byte, a 32 bit values
now fits 1 block including the metadata (length and id). This used to
be 3 blocks.

The data_length will occupy 1 byte if the data length is less than 128
byte, it will occupy 2 byte if the data length is 128 byte or more. The
data length is limited to 16383 byte.

Each write to flash is verified by a read back of the data.

The read performance is improved because reading is done backwards so
the latest items are found first.

When the filesystem is locked it can be unlocked by calling
reinit(), this will clear flash and setup everything for storage.

add sample documentation - README.rst

Update dtsi to include erase_block_size, use erase_block_size in sample

Update prj.conf to include CONFIG_MPU_ALLOW_FLASH_WRITE

Signed-off-by: Laczen JMS <laczenjms@gmail.com>