build,json: 3rd fixup of default_packages

build,json: 3rd fixup of default_packages

This became a bit of a tragedy, caused by a corner cases which wasn't
put into account during testing. DEFAULT_PACKAGES are defined in
target/linux/<target>/Makefile but also in
target/linux/<target>/<subtarget>/target.mk.

The latter was no longer imported when using DUMP=1, however not using
DUMP=1 while running the Makefile in target/linux/<target>/ caused duplicate
packages in the list.

As a solution, which should have been used from day 0, `make` runs in
target/linux/ without DUMP=1, resulting in no duplicate packages and all
inclusions from include/target.mk, linux/target/<target>/{Makefile,
<subtarget>/target.mk}

While at it, sort the list of default packages.

Signed-off-by: Paul Spooren <mail@aparcar.org>