Bluetooth: Mesh: Restructure Kconfig options

Current structure of Bluetooth Mesh Kconfig options is quite messy.
This makes impossible to understand which configuration belongs to
which feature. Especially when using interactive Kconfig interface, like
menuconfig or guiconfig.

This commit restructures the options grouping them by protocol layer
they belong to (Network, Transport, Access, etc.), or specific feature
(LPN, Friend, Proxy, Relay, etc.), or implementation (Advertiser, Shell,
Persistent storage). Amount of supported keys, subnets, group addresses
and labels are grouped under Capabilities menu. Generic options that
don't fall to any category are kept at the root menu. For better
visibility, if a specific feature or layer has more than 1 option, they
are hidden under menu.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>