Bug#816523: debian-installer: make fails if there's "APT::Default-Release" setting in system's apt.conf

[Roger Shimizu <rogershimizu@gmail.com>]

Package: src:debian-installer
Version: 20160112
Severity: normal
X-Debbugs-Cc: rogershimizu@gmail.com

Dear Maintainer,

build command "fakeroot make build_netboot" fails on my amd64 PC with the log:
(build on other platform such as armel also has similar issue)

====
E: The value 'jessie' is invalid for APT::Default-Release as such a
release is not available in the sources
Makefile:635: recipe for target 'stamps/get_udebs-netboot-stamp' failed
make[2]: *** [stamps/get_udebs-netboot-stamp] Error 100
Makefile:285: recipe for target '_build' failed
make[1]: *** [_build] Error 2
Makefile:279: recipe for target 'build_netboot' failed
make: *** [build_netboot] Error 2
====

my /etc/apt/apt.conf has the following setting:

  APT::Default-Release "jessie";

when I comment out the setting above, the build can be done without problem.
Although usually we build in unstable, I guess it's better to improve
this in some way.

Cheers,
-- 
Roger Shimizu, GMT +9 Tokyo
PGP/GPG: 17B3ACB1