Add an ability to mark a configuration as invalid

... so that the build will fail early and predictably.

Signed-off-by: Alexey Neyman <stilor@att.net>

config/global.in

@@ -38,6 +38,9 @@ config CONFIG_VERSION
     default "0" if VCHECK = "load" || VCHECK = "save"
     default CONFIG_VERSION_CURRENT
 
+config INVALID_CONFIGURATION
+    bool
+
 if VCHECK = "warning"
 comment "*************************************************************************"
 comment "Loaded configuration was generated by a previous version of crosstool-NG."

maintainer/kconfig-versions.template

@@ -276,6 +276,25 @@ config @@fork|@@_VERY_OLD
 #!end-foreach
 
 endchoice
+
+# A flag indicating that no valid selections exist for the version.
+# This may happen if there are conflicting requirements set by
+# the host system and/or other packages.
+config @@fork|@@_NO_VERSIONS
+    def_bool y
+    select INVALID_CONFIGURATION
+    depends on !@@fork|@@_VERY_NEW
+#!foreach version
+    depends on !@@fork|@@_V_@@ver_sel|@@ 
+#!end-foreach
+    depends on !@@fork|@@_VERY_OLD
+
+if @@fork|@@_NO_VERSIONS
+comment "WARNING!"
+comment "|  The @@master@@ package has no viable versions"
+comment "|  due to conflicting constraints."
+endif
+
 #!end-if
 
 #!if [ -n "@@versionlocked@@" ]

scripts/crosstool-NG.sh

@@ -32,6 +32,8 @@ if [ -z "${CT_ALLOW_BUILD_AS_ROOT_SURE}" ]; then
     fi
 fi
 
+CT_TestAndAbort "Invalid configuration. Run 'ct-ng menuconfig' and check which options select INVALID_CONFIGURATION." -n "${CT_INVALID_CONFIGURATION}"
+
 # If we want an interactive debug-shell, we must ensure these FDs
 # are indeed connected to a terminal (and not redirected in any way).
 if [ "${CT_DEBUG_INTERACTIVE}" = "y" -a ! \( -t 0 -a -t 6 -a -t 2 \) ]; then