* Partially revert a change made in 69ceb95ddc.
The change caused the 'libretro_directory' config option to always
revert to the default value, which was not intended behavior.
Reported-by: Michael Cook
* platform: Honor the LIBRETRO_DATABASE_DIRECTORY environment variable.
This is a follow-up to commit 69ceb95ddc.
* frontend/drivers/platform_unix.c
(libretro_database_directory): New variable.
(frontend_unix_get_env): Set DEFAULT_DIR_DATABASE to the value of
the LIBRETRO_DATABASE_DIRECTORY environment variable, if available.
* frontend/drivers/platform_win32.c: Likewise.
* configuration.c (config_load_file)
<libretro_database_directory>: New variable. Use the values of
the LIBRETRO_DATABASE_DIRECTORY environment variables instead of their
corresponding configured values, when set.
* docs/retroarch.6: Document.
* retroarch.c (retroarch_print_help): List supported environment
variables, and cross-reference the man page.
274cd419fa
