tar: bump to version 1.28

Also add hash file and set eprefix to / otherwise when busybox is around
we've got duplicate tar applications (/bin/tar for busybox, /usr/bin/tar
for full-blown) normally making busybox still win in directory search.

[Thomas: slightly improve the comment that explains why we pass
--exec-prefix.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>