pinentry: new package

pinentry: new package

[Thomas:
 - fix license version: the license is GPLv2+, not GPLv2
 - make sure the ncurses backend is selected if none of the other
   backends are selected.
 - add dependency on libiconv of the curses and gtk2 backends, needed
   in !locale configurations.]

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Markos Chandras <Markos.Chandras@imgtec.com>
Reviewed-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>