moveconfig: Support looking for implied CONFIG options

Some CONFIG options can be implied by others and this can help to reduce
the size of the defconfig files. For example, CONFIG_X86 implies
CONFIG_CMD_IRQ, so we can put 'imply CMD_IRQ' under 'config X86' and
all x86 boards will have that option, avoiding adding CONFIG_CMD_IRQ to
each of the x86 defconfig files.

Add a -i option which searches for such options.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>