cli: Consume invalid escape sequences early

Unexpected 'Esc' key presses are accumulated internally, even if it is
already clear that the current escape sequence is invalid. This results
in weird behaviour. For example, the next character after 'Esc' key
simply disappears from input and 'Unknown command' is printed
after 'Enter'.

This commit fixes some issues with extra 'Esc' keys entered by user:

1. Sequence <Esc><Esc><Enter> right after autoboot stop gives:
=>
nknown command 'ry 'help'
=>
2. Sequence <Esc><p><r><i><Enter> gives:
=> ri
Unknown command 'ri' - try 'help'
=>
3. Extra 'Esc' key presses break backspace functionality.

Signed-off-by: Yurii Monakov <monakov.y@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>