busybox：ash：PS1：支持的内部替换变量

网上资料没查到，那就找源码呗！

xxx\libbb\lineedit.c

/*
 * Supported via bb_process_escape_sequence:
 * \a    ASCII bell character (07)
 * \e    ASCII escape character (033)
 * \n    newline
 * \r    carriage return
 * \\    backslash
 * \nnn    char with octal code nnn

 * Supported:
 * \$    if the effective UID is 0, a #, otherwise a $
 * \w    current working directory, with $HOME abbreviated with a tilde
 *    Note: we do not support $PROMPT_DIRTRIM=n feature
 * \W    basename of the current working directory, with $HOME abbreviated with a tilde
 * \h    hostname up to the first '.'
 * \H    hostname
 * \u    username
 * \[    begin a sequence of non-printing characters
 * \]    end a sequence of non-printing characters
 * \T    current time in 12-hour HH:MM:SS format
 * \@    current time in 12-hour am/pm format
 * \A    current time in 24-hour HH:MM format
 * \t    current time in 24-hour HH:MM:SS format
 *    (all of the above work as \A)
 * Not supported:
 * \!    history number of this command
 * \#    command number of this command
 * \j    number of jobs currently managed by the shell
 * \l    basename of the shell's terminal device name
 * \s    name of the shell, the basename of $0 (the portion following the final slash)
 * \V    release of bash, version + patch level (e.g., 2.00.0)
 * \d    date in "Weekday Month Date" format (e.g., "Tue May 26")
 * \D{format}
 *    format is passed to strftime(3).
 *    An empty format results in a locale-specific time representation.
 *    The braces are required.
 * Mishandled by bb_process_escape_sequence:
 * \v    version of bash (e.g., 2.00)
 */