Currently, the list of external trees is a private variable, but for
debugging or compliance, one may need to get that list.
Add a Kconfig option so that the list appears in the .config file, and
export the already existing .mk variable in the environment, so that
post-build or post-image scripts can use it.
Reported-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
8a2b772e8c