uboot-tools: add option for mkenvimage

The tool mkenvimage generates a valid binary environment image from
a text file describing the key=value pairs of the environment.
This commit allows installation on target and host.

Signed-off-by: Arnaud Rébillout <rebillout@syscom.ch>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>