core/pkg-generic: sanitise FOO_OVERRIDE_SRCDIR

In case FOO_OVERRIDE_SRCDIR has trailing spaces, like so:
    FOO_OVERRIDE_SRCDIR = /path/to/sources\x20

we would end up with a rsync command like so:
    rsync -au /path/to/sources / /path/to/build/foo

which would effectively rsync the whole vfs, eventually filling the
whole disk... :-(

So, just qstrip the variable before use.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>