Added 'ftp_proxy' environment variable support

Some bitbake recipes use FTP to fetch objects. This patch adds support
for the ftp_proxy variable to kas. Is now usable in the same manner as
the 'http_proxy' variable

Signed-off-by: Claudius Heine <ch@denx.de>

kas/configschema.py

@@ -161,6 +161,9 @@ CONFIGSCHEMA = {
                 'https_proxy': {
                     'type': 'string',
                 },
+                'ftp_proxy': {
+                    'type': 'string',
+                },
                 'no_proxy': {
                     'type': 'string',
                 },