efi_loader: efi_console: support editable input fields

efi_loader: efi_console: support editable input fields

When editing eficonfig "optional data" (typically cmdline arguments)
it's useful to be able to edit the string rather than having to re-type
the entire thing. Implement support for editing buffers to make this a
whole lot nicer. Specifically, add support for moving the cursor with
the arrow keys and End key as well as deleting backwards with the delete
key.

Signed-off-by: Casey Connolly <casey.connolly@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>