android: boot: support bootconfig

Support Bootconfig feature.
- The bootconfig feature replaces the androidboot.*
  kernel cmdline options.

This was adapted from downstream [1] commit : 7af0a0506d4d ("cuttlefish:
support bootconfig parameters").

Link:[1] https://android.googlesource.com/platform/external/u-boot/



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