bootscript[-alt|-uboot]: place boot.cfg and variants into deploy directory

boot.cfg and similar files are installed into /boot folder of rootfs.
For the rauc bootfs-image and the WIC image they are also needed in the
deploy folder and the IMAGE_BOOT_FILES variable.
As the selection of the specific bootfiles is already done using the
packages, the IMAGE_BOOT_FILES variable is configured to install
everything from the boot/ subdir in the deploy path and the packages
deploy the needed scripts there.

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>