tools: binman: add 'fit, encrypt' property to pass keys directory to mkimage

tools: binman: add 'fit, encrypt' property to pass keys directory to mkimage

mkimage can be used for both signing the FIT or encrypt its content and the
option '-k' can be used to pass a directory where both signing and encryption
keys can be retrieved. Adding 'fit,encrypt' property to the 'fit' node, leads to
try to find keys directory among binman include directories.
_get_priv_keys_dir() is renamed as _get_keys_dir() and adapted to support both
signing and encryption nodes in the FIT.

Signed-off-by: Paul HENRYS <paul.henrys_ext@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>