core: split generated kconfig file

core: split generated kconfig file

Currently, the kconfig part contains two things: the kconfig option
with the paths to br2-external trees, and the kconfig menus for the
br2-external trees.

When we want to include more kconfig files from the br2-external tree
(e.g. to get definitions for pre-built toolchains), we will need to
have the paths defined earlier, so they can be used from the br2-external
tree to include files earlier than the existing menus.

Split the generated kconfig file in two: one to define the paths, which
gets included early in our main Config.in, and one to actually define
the existing menus, which still gets included at the same place they
currently are.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>