Fixing bug in line editing of the shell with CTRL-U.

This patch fixes a tiny bug in the command line editing of the RTEMS shell.
Typing CTRL-U in the shell should remove all characters left of the cursor.
After pressing CTRL-U, the current implementation does wrongly place the cursor
at the end of the line instead at its beginning.

To reproduce the bug, start the shell and type 'abc123' (no <RETURN>):

> ~/src/rtems $ qemu-system-arm -net none -nographic -M realview-pbx-a9 \
  -m 256M -kernel build/arm/realview_pbx_a9_qemu/testsuites/libtests/dl10.exe
> *** BEGIN OF TEST libdl (RTL) 10 ***
> *** TEST VERSION: 6.0.0.d9bdf166644f612dd628fe4951c12c6f8e94ba5f
> *** TEST STATE: USER_INPUT
> *** TEST BUILD: RTEMS_DEBUG RTEMS_NETWORKING RTEMS_POSIX_API RTEMS_SMP
> *** TEST TOOLS: 10.2.1 20200904 \
  (RTEMS 6, RSB 31f936a7b74d60bda609a9960c6e1a705ba54974, Newlib a0d7982)
> RTL (libdl) commands: dl, rtl
>
> RTEMS Shell on /dev/foobar. Use 'help' to list commands.
> SHLL [/] # abc123

Then move the cursor onto the '1' by hitting three times the <ARROW-LEFT> key.
Next type <CTRL>-U:

> SHLL [/] # 123

Note that the cursor is at the end of the line (after '3') instead of correctly
at the beginning (on the '1'), now.

Continuing typing 'echo ' incorrectly results in the output:

> SHLL [/] # 123echo 123

The patch changes this behavior so that the cursor in the second last step will
be on the '1' and typing 'echo ' will then correctly reflected as:

> SHLL [/] # echo 123

Close #4097.

0 files changed, 0 insertions, 0 deletions