package: fix 'local' site method for host packages

Using the 'local' site method works just fine for target
packages. However, for host packages, when HOST_<pkg>_SITE is
automatically defined by the package infrastructure to be equal to
<pkg>_SITE, when defining the <pkg>_OVERRIDE_SRCDIR, the $($(2)_SITE)
is empty, due to a missing additional dollar sign.

This patch ensures that the <pkg>_OVERRIDE_SRCDIR gets the correct
value, regardless of whether the HOST_<pkg>_SITE variable has been
defined by the package itself, or inferred by the package
infrastructure using the <pkg>_SITE value.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by: http://stackoverflow.com/questions/19311747/buildroot-cant-use-local-site-method-for-custom-host-packages
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-generic.mk

@@ -281,7 +281,7 @@ endif
 
 ifeq ($$($(2)_SITE_METHOD),local)
 ifeq ($$($(2)_OVERRIDE_SRCDIR),)
-$(2)_OVERRIDE_SRCDIR = $($(2)_SITE)
+$(2)_OVERRIDE_SRCDIR = $$($(2)_SITE)
 endif
 endif