blobs: Introduce TAINT and TAINT_BLOBS Kconfig options

In order to ensure that builds properly reflect the use of binary blobs,
it is important to markt the build as tainted. For that purpose
introduce two new Kconfig options that ultimately will be reflected in
the build image itself.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>