configuration access code refactoring

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[config] loader configuration parsing

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[loader] Provider C++ Loader API

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[cmake] STATEFS_LIB_DIR preprocessor definition

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[loader] using default loader

Now path is hard-coded, adding loaders registration later

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[config] provider metainfo: type

type is used to detect which loader should be used

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[loader] using provider loaders

Provider loader configuration accessing/registration, provider is loaded
using loader, depending on command-line option --statefs-type etc.

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>