If GNU wget is available, then we want use it as it gives the user
possibility to set misc options, such as bandwidth limit in wgetrc.

It might also be that busybox is not available in case bootstrapping
alpine from other distros (think debian vserver host creating an
alpine guest)

In any case we, fall back to busybox wget in case wget libs are getting
upgraded. (see http://redmine.alpinelinux.org/issues/347)