core-image-minimal-initramfs: Use PACKAGE_INSTALL instead of IMAGE_INSTALL
PACKAGE_INSTALL is set from the IMAGE_INSTALL variable with some additional
items, since PACKAGE_INSTALL is intended to be more an internal variable, use
it instead of the IMAGE_INSTALL which is recommeded for installing additional
packages. This will allow the initramfs recipe to use a fixed set of packages
and not be affected by IMAGE_INSTALL.
