To build U-Boot on a Nyan Big Chromebook the docs outline adjusting the Tegra124
defined CONFIG_SYS_TEXT_BASE but this has since been moved to individual config
files. We should have the default required for U-Boot chain loading on the
chromebook as the default CONFIG_SYS_TEXT_BASE and update the docs to remove
this now non required step.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>

Add instructions for chromebook_jerry.

Signed-off-by: Simon Glass <sjg@chromium.org>

Most Chromebooks support chain-loading U-Boot but instructions are
somewhat scattered. Add a README to hold this information within the
U-Boot tree. Also add the standard developer keys to simplify the
instructions, since they are small.

For now this only supports nyan-big.

Signed-off-by: Simon Glass <sjg@chromium.org>