binman: add sign option for binman

binman: add sign option for binman

Introduce proof of concept for binman's new option which provides sign
and replace FIT containers in binary images.

Usage as example:

from:
mkimage -G privateky -r -o sha256,rsa4096 -F fit
binman replace -i flash.bin -f fit.fit fit

to:
binman sign -i flash.bin -k privatekey -a sha256,rsa4096 -f fit.fit fit

and to this one if it's need to be extracted, signed with key and put it
back in image:
binman sign -i flash.bin -k privatekey -a sha256,rsa4096 fit

Signed-off-by: Ivan Mikhaylov <fr0st61te@gmail.com>