When booting a Raspberry Pi, it is useful to extract bootargs from the
device tree that was created by the VideoCore firmware. These bootargs
contain for example settings for the framebuffer that the kernel needs
to properly set the video output.

This commit extracts the bootargs in the board initialization code and
saves them to the vc.bootargs global variable.

For example, a bootloader environment can then add the contents of this
variable to linux.bootargs.vc, which then gets included into the final
bootargs for the kernel using CONFIG_FLEXIBLE_BOOTARGS.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>