samples: mesh: nrf52: storing of some Prime states on SoC flash

With this commit it is now possible to save some Mesh Model
Servers State on SoC flash. This impacts initialization &
state binding after NODE reset & that is why some additional
modification has done as per requirements & removed some
code & comments which are redundant as per current
implementation.

Signed-off-by: Vikrant More <vikrant8051@gmail.com>