package/tzdata: install a default localtime

Add an option to install a default localtime.

since tzdata only makes sense for (e)glibc, we don't care
that we install timezone files that uClibc can't understand.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>