sys/shell: ensure character is flushed when echoing.

When using a serial terminal without local echo, the current line
would not get updated as the user typed because the shell module's
readline() was not flushing each character.

This commit fixes that behavior. For additional clarity, fflush is
turned into a macro (flush_if_needed) which expands to either a call
to fflush() or empty, according to the standard library used.

This also fixes the erase/line editing behavior (the delete characters
were not being flushed either.)