config: add init_targets proc that is executed just before init

config: add init_targets proc that is executed just before init

this allows configuration scripts to export a init_targets proc
rather than setting up the target directly.

This allows for new conventions in how to set up target vs. board
script and how to transfer default settings between board and
target scripts.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>