exynos5: Use config_distro_bootcmd.h

exynos5: Use config_distro_bootcmd.h

This replaces the existing CONFIG_BOOTCOMMAND for exynos5250 and 5420.

exynos4 platforms seem to have existing complex extra env configuration for
booting and so are excluded here. Hence the bootcmd.h is added to
exynos5-common.h.

I have build tested on all exynos platforms (MAKEALL -s exynos), but only boot
tested on arndale.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Guillaume GARDET <guillaume.gardet@free.fr>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>