cmd: bootmenu: rename U-Boot console to Exit

cmd: bootmenu: rename U-Boot console to Exit

It seems that the U-Boot console entry of the bootmenu has lost
its original meaning. Now, even if it is chosen, the probability
that you will enter the actual U-Boot console is quite low.
Boot env, bootflow, bootcommand script may appear, but not the
actual console. Hence, let's remove ambiguity and name this
entry by what it actually does: 'Exit' the bootmenu.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>