binman: implement signing FIT images during image build

binman: implement signing FIT images during image build

The patch implement new property 'fit,sign' that can be declared
at the top-level 'fit' node. If that option is declared, fit tryies
to detect private keys directory among binman include directories.
That directory than passed to mkimage using '-k' flag and that enable
signing of FIT.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Renumbered files, moved new tests to end:
Signed-off-by: Simon Glass <sjg@chromium.org>