atftp: fix static linking

Readline uses ncurses hence pull it in, also needs to explicitly link
against libpthread. Fixes:
http://autobuild.buildroot.net/results/259/2592612be38a2c1dde21b44be2ecf25d1cc5211a/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>