infra: remove incorrect default for FOO_DIR_PREFIX

Variable FOO_DIR_PREFIX is populated from pkgparentdir by the various
package infrastructures. However, if that would be empty (which in fact is
the case for the linux package), FOO_DIR_PREFIX would be set to
'$(TOP_SRCDIR)/package'.
Not only does this make no sense (LINUX_DIR_PREFIX becomes /package/linux,
and for all other packages pkgparentdir is not-empty anyway), but it is also
using a non-existing variable TOP_SRCDIR.
This patch therefore removes the incorrect default.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>