package: enhance infrastructure to support source dir override

package: enhance infrastructure to support source dir override

When a variable <pkg>_OVERRIDE_SRCDIR is defined, then Buildroot will
no longer try to download, extract and patch the package. It will
simply use the value of this variable as the source directory for the
package. The contents of the package sources will be rsynced to the
package build directory in $(O)/build/pkg-version/.

This can be used to tell Buildroot that the sources for a given
package are inside some directory that you control, and which can be
versioned in Git/SVN, or handled in whichever way you want.

Those <pkg>_OVERRIDE_SRCDIR variables will be defined by a local
makefile included by Buildroot, which will be handled in a later
commit.

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