[lwm2mcore] Compile all tinydtls within externalBuild section
[lwm2mcore] Compile all tinydtls within externalBuild section

Resolves: LE-7075
Change-Id: I6665cef2faf731a4df49da3d086054aa7cbe06f7

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af