build: introduce support to declare skip package

build: introduce support to declare skip package

It seems some target started declaring package in DEVICE_PACKAGES just
to call InstallDev and generate binary for the image firmware.

This is very much used by layerscape target where trusted-firmware-a and
dependency are called for final image generation.

This is problematic for APK since it's more sensible to non exisiting
package.

To handle this, introduce a prefix '~' for a package that will signal to
build the package but not install it in the final image.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>