Kconfig: rename folder to kconfigs

On case-insensitive file systems the root Kconfig file clashes with the
kconfig folder where we keep constants and features, producing the
deletion of one of them. This renames the folder to remove the conflict.

Author: leandrolanzieri

Kconfig

@@ -7,10 +7,10 @@
 mainmenu "RIOT Configuration"
 
 # Load macro definitions
-rsource "kconfig/Kconfig.consts"
+rsource "kconfigs/Kconfig.consts"
 
 # Load feature declarations
-rsource "kconfig/Kconfig.features"
+rsource "kconfigs/Kconfig.features"
 
 # For now, get used modules as macros from this file (see kconfig.mk)
 osource "$(KCONFIG_GENERATED_DEPENDENCIES)"