core: move generated .br2-external kconfig file to $(BASE_DIR)

core: move generated .br2-external kconfig file to $(BASE_DIR)

Currently, that file is generated rather late in the configuration
process, so BUILD_DIR is known (and exists) by then.

We're soon to generate that file much earlier, at a point where
BUILD_DIR is not yet known, so we have two options:
 1- declare BUILD_DIR earlier;
 2- generate the file in an already-known location.

We go with the second solution, as we're already generating a
br2-external related file in BASE_DIR, so we can as well generate all
br2-external files in the same place.

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>