Perform a simple rename of CONFIG_EXTRA_ENV_SETTINGS to CFG_EXTRA_ENV_SETTINGS

Signed-off-by: Tom Rini <trini@konsulko.com>

This adds keyword devicetree-overlay as an alias for fdtoverlays in
extlinux (sysboot) and pxe to better follow the Boot Loader Specification
[1], improves documentation around them by adding an example for both
fdtoverlays and devicetree-overlay and the environment variable required
for this feature. The link for the spec is updated to the current one.

[1] https://systemd.io/BOOT_LOADER_SPECIFICATION/

Signed-off-by: Edoardo Tomelleri <e.tomell@gmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>

Add documentation for this feature, including the commands and full
devicetree bindings.

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

Add HOST, UBIFS.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

Move this over to the new rST format.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Artem Lapkin <email2tema@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>