buildroot

configs/warp7: Bump U-Boot and kernel versions

Bump U-Boot to 2017.07 and kernel to version 4.12.3.

As imx_v6_v7_defconfig in 4.12 selects CONFIG_CFG80211_WEXT=y,
remove the linux.fragment file.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>