This commit changes the method of accessing the default v6 prefix. Instead of always using `UIP_DS6_DEFAULT_PREFIX`, we store the default prefix in a variable. We subsequently retrieve the prefix by calling `uip_ds6_default_prefix()`.
`uip_ds6_init()` will set this variable to the value of `UIP_DS6_DEFAULT_PREFIX`, but only if the startup code has not set a different default prefix before `uip_ds6_init()` gets called.
This approach has the following benefits:
* It allows us to change the default prefix at run time.
* It allows the startup code to set a prefix different than the one specified by `UIP_DS6_DEFAULT_PREFIX`, which can be useful if the default prefix comes from a different source (e.g. the command line for native, or a configuration stored on a node's flash)
* In many places the current code assumes that the prefix contains 6 bytes of zeros (e.g. `FDxx::/64`). Changing to a different prefix (e.g. `FD00:ABCD::`) would need extensive code changes. This change here makes it easy to use a prefix of any length.
