infra: remove variable FOO_DIR_PREFIX

Variable FOO_DIR_PREFIX in inner-generic-package isn't really needed. The
contents of this variable are 'package' for normal packages, 'boot' for
bootloaders, and 'linux' for the linux kernel.
When patching a package, all you need to know is the directory where
patches can reside, which is already returned by $(pkgdir). In order to be
able to use this variable outside of inner-generic-package, we introduce a
target-specific variable PKGDIR that equals to this $(pkgdir).

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>