Revert "arm: config: fix default console only to specify the device"

This reverts commit 232ed3ca.

In exynos boards, ${console} is used to set bootargs but it sets
without "console=", so CONFIG_DEFAULT_CONSOLE for these boards is
designated with "console=" but it is removed. So revert the commit.

References for using ${console} in
   board/samsung/common/bootscripts/autoboot.cmd
   board/samsung/common/bootscripts/bootzimg.cmd

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>