package: implement a 'local' site method

The new override source directory mechanism allows to tell Buildroot
to use a particular directory as the source directory for a
package. However, this mechanism works with a local override makefile
and not directly within the package recipe itself.

For some use cases, it might be desirable to write a package recipe
which always refers to a local source directory (and not a http, git,
svn or bazaar download). This commit makes this possible by adding the
'local' site method. It allows to write package recipes as follows:

MYPKG_SITE = /tmp/mypkg-source-code
MYPKG_SITE_METHOD = local

[...]

$(eval $(call GENTARGETS,package,mypkg))

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>