buildroot

configs/stm32f4xx: use kernel version 4.11

Use kernel 4.11 for stm32f4 discovery board
(stm32f429i-disco and stm32f469i-disco).

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>