pkg-generic: substitute colon and space out of PKG_VERSION

Using a colon or a space in a make target doesn't work, so they have to
be filtered out of the PKG_VERSION variable just like the / currently
already is.

This will be needed for date-based CVS versions.

Add a helper macro 'sanitize' in pkg-utils.mk to implement this. This
also moves the strip to the inner level for the target branch.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>