device-nvidia-tegra-armv7: remove console entry

after !5000 (closed) was merged, when I updated my tegra-armv7 devices (asus-tf201 and asus-grouper), these devices would not boot into the graphical UI.

removing the console=... from the kernel cmd line in the extlinux.conf fixes this.

please only merge after testng on other tegra-armv7 devices!