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>

Kconfig.zephyr

@@ -630,6 +630,12 @@ config WARN_EXPERIMENTAL
 	  Print a warning when the Kconfig tree is parsed if any experimental
 	  features are enabled.
 
+config TAINT
+	bool
+	help
+	  Symbol that must be selected by a feature or module if the Zephyr
+	  build is considered tainted.
+
 config ENFORCE_ZEPHYR_STDINT
 	bool
 	prompt "Enforce Zephyr convention for stdint"

modules/Kconfig

@@ -1,6 +1,13 @@
 # Copyright (c) 2019 Intel Corporation
 # SPDX-License-Identifier: Apache-2.0
 
+config TAINT_BLOBS
+	bool
+	select TAINT
+	help
+	  This option is selected when binary blobs are present locally at
+	  build time to reflect that the build might have been tainted by them.
+
 comment "Available modules."
 
 osource "$(KCONFIG_BINARY_DIR)/Kconfig.modules"