pkg-generic.mk: don't depend on external package permissions

Reset permissions for rsynced packages (when using OVERRIDE_SRCDIR) to
755/644. We do this under the assumption that source files shouldn't
care about their permissions, except possibly for the exec bit.

This guarantees that if a package uses 'rsync -a' or 'cp -p' to copy
a file from its build dir to the target, it'll end up with the same
permissions on the target every time.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>