pkg-generic: error out with 'local' site method and no _SITE

pkg-generic: error out with 'local' site method and no _SITE

The 'local' site method is easily confused with the 'file' site method,
making people create packages like this:

    FOO_SITE_METHOD = local
    FOO_SOURCE = foo.tar.gz

    $(eval $(generic-package))

Due to the intricacies of the generic package infra, this does not
cause an error; instead, the foo.tar.gz tarball that happens to be
present in the download directory will be used. This behaviour differs
greatly from what is specified in the manual.

Instead, error out immediately if a package specifies the 'local' site
method but does not specify a _SITE.

We check for _OVERRIDE_SRCDIR rather than checking for _SITE, just
after _OVERRIDE_SRCDIR has been set to _SITE. Indeed, a package that
sets _OVERRIDE_SRCDIR but not _SITE currently works correctly. There is
no reason to make it fail.

See also
https://stackoverflow.com/questions/50364655/including-patches-to-build-root

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>