ARM: 7437/1: zImage: Allow DTB command line concatenation with ATAG_CMDLINE

This patch allows the ATAG_CMDLINE provided by the bootloader to be
concatenated to the bootargs property of the device tree.

This is useful to merge static values defined in the device tree
with the boot loader's (possibly) more dynamic values, such as
startup reasons and more.

The bootloader should use the device tree to pass those values to
the kernel, but that's not always simple (old bootloader or very
small one).

The behaviour is the same as the one introduced by Victor Boivie in
4394c124 by extending the CONFIG_CMDLINE.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>