core: commonalise the bundled and br2-external %_defconfig rules

core: commonalise the bundled and br2-external %_defconfig rules

The code for both cases is exactly the same, and only differs in the
location where defconfig files are looked for.

We use an intermediate macro to generate the corresponding rules,
because directly generating the rules is ugly and needs lots of escaping
and double-dollar-ing for the $(eval ...) and $(foreach ...) calls to
play nicely together.

Furthermore, that will be tremendously useful when we support multiple
br2-external trees.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
[Thomas: move comment outside of the make target, so that it isn't
displayed on stdout when loading a defconfig.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>