buildroot

board: add stm32f429-disc1 config

This change renders the stm32f429discovery a bit more generic and reuse most
of its settings for the stm32f429-disc1 board.

Cc: Waldemar Brodkorb <wbx@openadk.org>
Cc: Davide Viti <zinosat@tiscali.it>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>