android: boot: support extra command line

In version 3 and 4 of boot image header, the vendor specific
command line are located in vendor boot image. Thus, use
extra command line to add those cmd to bootargs.

Signed-off-by: Safae Ouajih <souajih@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>