uboot: integrate mkenvimage

This commit provides configuration options to automatically generate a
binary environment image for U-Boot.
Two options are available (and mandatory):
 * the location of a text file describing U-Boot environment.
 * the size of the environment.

[Peter: fix Config.in indentation + typo]
Signed-off-by: Arnaud Rébillout <rebillout@syscom.ch>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>