buildroot

samba4: bump to version 4.1.7 and improve cross build

Bump to the latest 4.1.7 version and improve the cross-build logic.
With the new patches the build is basically architecture-agnostic making
it possible to ditch the arch-specific cache to use a generic one.
Some toolchains might not be too happy with samba4 because of bitrot,
hopefully we'll find and fix or blacklist those with autobuilder help.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>