configs/toradex_apalis_imx6: bump U-Boot and kernel to the official image version 2.7

According to the following guide:
https://developer.toradex.com/knowledge-base/build-u-boot-and-linux-kernel-from-source-code#iMX_6_Based_Modules_ApalisColibri_iMX6

The official final 2.7 version released at 2018-01-04, see the following link:
https://developer.toradex.com/software/linux/linux-software/release-details



Signed-off-by: Scott Fan <fancp2007@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>