buildroot

core: move rule to create basic directories

Some of those directories will be needed even during configuration, like
BUILD_DIR, where we'll store the generated kconfig snippet.

So, move the rule to create them outside the BR2_HAVE_DOT_CONFIG block.

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>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
[Thomas: fixup commit log, as noticed by Romain Naour.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>