confserver: Always store hex values in sdkconfig with 0x prefix

This is not necessary for correct behaviour or to have valid sdkconfig files
(previous commit adds tests for this), but it's useful for consistency with
sdkconfig files generated by menuconfig.

As reported in https://github.com/espressif/vscode-esp-idf-extension/issues/83

tools/kconfig_new/confserver.py

@@ -228,7 +228,7 @@ def handle_set(config, error, to_set):
                 try:
                     if not isinstance(val, int):
                         val = int(val, 16)  # input can be a decimal JSON value or a string of hex digits
-                    sym.set_value("%x" % val)
+                    sym.set_value(hex(val))
                 except ValueError:
                     error.append("Hex symbol %s can accept a decimal integer or a string of hex digits, only")
             else: