uhd: paths: Harmonize around XDG Base Directory specification

Up until now, we completely ignore the XDG specification.

This commit does the following to change that:

- It uses XDG_DATA_HOME and XDG_CONFIG_HOME for cal and config data,
  respectively.
- If config data is in ~/.uhd/uhd.conf, that is still accepted, but if
  it conflicts with $XDG_CONFIG_HOME/uhd.conf, it is ignored and a
  warning is displayed
- The default location for cal data is thus ${HOME}/.local/share/uhd/cal
  on Unix, and %LOCALAPPDATA%\uhd\cal on Windows. This is a change in
  location!
- The UHD_CONFIG_DIR environment variable was confusingly named and is
  now removed. It provided an alternative location than the home
  directory. The same purpose is now much better served by XDG_DATA_HOME
  and XDG_CONFIG_HOME.

The specification can be found here:
specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

1 files changed, 3 insertions, 3 deletions

diff --git a/host/docs/configfiles.dox b/host/docs/configfiles.dox
index bc210eba9..f32274e73 100644
--- a/host/docs/configfiles.dox
+++ b/host/docs/configfiles.dox
@@ -55,9 +55,9 @@ directly.
    <b><tt>\%programdata%/uhd/uhd.conf</tt></b>. The default can be overwritten
    at compile time by setting `UHD_SYS_CONF_FILE` through CMake.
 2. A user-specific configuration file. On Unix systems, it defaults to
-   `$APPDATA/.uhd/uhd.conf`, where `$APPDATA` is usually the same as the user's
-   home directory, unless `$APPDATA` or `$USER_CONFIG_DIR` are defined. On
-   Windows systems, it simply resolves to <b><tt>\%appdata%</tt></b>.
+   `$XDG_CONFIG_HOME/uhd.conf`, where `$XDG_CONFIG_HOME` is usually the same as
+   `$HOME/.config`. On Windows systems, it also checks
+   <b><tt>\%localappdata%</tt></b> and <b><tt>\%appdata%</tt></b>.
    The default can be overwritten at compile time by setting
    the `UHD_USER_CONF_FILE` through CMake.
 3. A configuration file specified by the environment variable `UHD_CONFIG_FILE`