samples: net: lwm2m_client: Allow configuration in Kconfig

Allow endpoint name, PSK and TLS tag and server address
to be configured in Kconfig.

When endpoint name is not defined, use CONFIG_BOARD.
Also use endpoint name as a PSK ID instead of hard coded
"Client_identity" which would collide if more than one instance
is ran against same server. Now we could define different identity
for each endpoint.

Also, as a mininal refactor, allow DNS names to be used in
server address, instead of assuming IP address.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>