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/btool/mkimage.py b/tools/binman/btool/mkimage.py
index 78d3301..3f84220 100644
--- a/tools/binman/btool/mkimage.py
+++ b/tools/binman/btool/mkimage.py
@@ -22,7 +22,7 @@
 
     # pylint: disable=R0913
     def run(self, reset_timestamp=False, output_fname=None, external=False,
-            pad=None, align=None, priv_keys_dir=None):
+            pad=None, align=None, keys_dir=None):
         """Run mkimage
 
         Args:
@@ -34,7 +34,7 @@
                 other things to be easily added later, if required, such as
                 signatures
             align: Bytes to use for alignment of the FIT and its external data
-            priv_keys_dir: Path to directory containing private keys
+            keys_dir: Path to directory containing private and encryption keys
             version: True to get the mkimage version
         """
         args = []
@@ -46,8 +46,8 @@
             args += ['-B', f'{align:x}']
         if reset_timestamp:
             args.append('-t')
-        if priv_keys_dir:
-            args += ['-k', f'{priv_keys_dir}']
+        if keys_dir:
+            args += ['-k', f'{keys_dir}']
         if output_fname:
             args += ['-F', output_fname]
         return self.run_cmd(*args)