settings: Moving initialization code to proper backend sources

Backend initialization code has been moved from common settings_init.c
to proper backend source files. Missing static specifiers have been
added.
Minor cleanup has been done to source files: exported functions have
been moved to the end of source files and definitions of static
variables, that are used by only a single function, have been moved from
global scope into functions that use them.

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