net: lwm2m: Fix TLS_HOSTNAME is always set

LWM2M Client Sample with DTLS enabled fails because TLS_HOSTNAME is set
but MBEDTLS_X509_CRT_PARSE_C is disabled which leads to error
'net_lwm2m_engine: Failed to set TLS_HOSTNAME option: 109'
Add new field hostname_verify to let the application decide if hostname
should be checked.

Signed-off-by: Benjamin Bigler <benjamin.bigler@securiton.ch>