board: simplify atngw100 support

Minimize atngw100_defconfig, remove atngw100-base_defconfig, and
remove the target skeleton and device table. Instead of having
complete copies of new target skeletons (making them hard to
maintain), we should just have a post-build script that
adds/removes/tweaks the existing target skeleton.

Moreover, most of the tweaks in this target skeleton were for specific
packages, but the policy now is that board defconfig should just build
a basic root filesystem with Busybox, and let the user select
whichever set of packages (s)he wants.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>