uboot-imx

MLK-18641 arm: Remove the check for gcc6 requirement

The u-boot 2018 forces to use gcc 6.0 or later to save binary size for some platforms.
This check causes problem for android building because android tool chain remains
using gcc 4.9.
Since iMX6/7/8 platforms don't meet binary size issue, we can remove the checkgcc6
and use old gcc.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 518b585b37cb79a52a558b9fc03fd9311855dab9)