boot: CONFIG_CEDIT must depend on CONFIG_EXPO

Building sandbox_defconfig with

    CONFIG_CMD_CEDIT=y
    CONFIG_EXPO=n

fails with

    cmd/cedit.c:258:(.text.do_cedit_run+0x4c):
    undefined reference to `expo_apply_theme

Fix the dependencies.

Fixes: a0874dc4 ("expo: Add a configuration editor")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>