boards: Kconfig: Enable use of shield conditional Kconfig files

To make use of shields Kconfig.defconfig, parse these files
and newly created Kconfig.shield files which will define the
SHIELD_XXX Kconfig symbols that will be used for conditional
configuration.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>

Kconfig.zephyr

@@ -17,9 +17,12 @@ endmenu
 # (e.g. the first default) with a satisfied condition.
 #
 # Board defaults should be parsed before SoC defaults, because boards usually
-# overrides SoC values.
+# overrides SoC values. Similarly shields defaults should be parsed before
+# boards defaults, because shields defaults override boards defaults.
 #
 # Note: $ARCH and $BOARD_DIR might be glob patterns.
+source "boards/shields/*/Kconfig.defconfig"
+
 source "$(BOARD_DIR)/Kconfig.defconfig"
 
 source "$(SOC_DIR)/$(ARCH)/*/Kconfig.defconfig"

boards/Kconfig

@@ -23,6 +23,9 @@ source "$(BOARD_DIR)/Kconfig.board"
 
 endchoice
 
+# Parse shields references
+# Don't do it as a menuconfig, as shield selection is a CMake feature.
+rsource "shields/*/Kconfig.shield"
 
 menu "Board Options"
 # There might not be any board options, hence the optional source