binman: add tests for sign option

Add the test which provides sequence of actions:
1. create the image from binman dts
2. create public and private keys
3. add public key into dtb with fdt_add_pubkey
4. 1. sign FIT container with new sign option with extracting from
image
2. sign exact FIT container with replacing of it in image
5. check with fit_check_sign

Signed-off-by: Ivan Mikhaylov <fr0st61te@gmail.com>
Renumber test file from 277 to 280;
Move UpdateSignatures() to Entry base class;
Don't allow missing mkimage as it doesn't make sense;
Propagate --toolpath for CI;
Call mark_build_done() to avoid regenerating FIT:
Signed-off-by: Simon Glass <sjg@chromium.org>