trivial: update buildroot e-mail address to buildroot@buildroot.org

As the e-mail address buildroot@buildroot.org is now enabled, update the
e-mail addresses in the source tree from @uclibc.org and @busybox.net to our
own proper domain.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>