u-boot: Restore valid default environment for running system

One significant change from previous OE-Core releases was the move from
u-boot-fw-utils to libubootenv which offers a generic and not machine
specific alternative. However, it is not fully functional as currently
we don't provide the default environment nor the required configuration
file to be used by it.

This change address this and include on the default u-boot.inc the
needed code to generate, install and deploy the default environment.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>