cmd: bootm: add subcommand preload

Add a subcommand preload to bootm that execute the preload
stage on the image. Right now, it checks the signature
of the image with the pre-load header. If the check
succeed, the u-boot env variable 'loadaddr_verified'
is set to the address of the image (without the header).

It allows to run such commands:
tftp script.img && bootm preload $loadaddr && source $loadaddr_verified

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>