Busybox df, ls and ash applet small improvements

Hello.

It seems that busybox df and ls applets don't respect BLOCK_SIZE=human environment variable.
With that variable there is no need to always use df -h or ls -h (or even any aliases for them).

Also the ash applet does not seem to respect the following backslash-escaped special characters in PS1 environment variable:

\d: the date in "Weekday Month Date" format (e.g., "Tue May 26") \t: the current time in 24-hour HH:MM:SS format
\T: the current time in 12-hour HH:MM:SS format
\@: the current time in 12-hour am/pm format
\A: the current time in 24-hour HH:MM format

Best regards:
Stefan