wic: bootimg-efi: implement --include-path

--include-path can be used for placing files on ESP,
like dtbs.

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>