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>
diff --git a/tools/binman/entries.rst b/tools/binman/entries.rst
index e918162..53024ac 100644
--- a/tools/binman/entries.rst
+++ b/tools/binman/entries.rst
@@ -871,6 +871,13 @@
         -k flag. All the keys required for signing FIT must be available at
         time of signing and must be located in single include directory.
 
+    fit,encrypt
+        Enable data encryption in FIT images via mkimage. If the property
+        is found, the keys path is detected among binman include
+        directories and passed to mkimage via  -k flag. All the keys
+        required for encrypting the FIT must be available at the time of
+        encrypting and must be located in a single include directory.
+
 Substitutions
 ~~~~~~~~~~~~~