toolchain-external: add GCC 6.x option

Commit 519d83bfa0442439026db4e71e605a5c1347e566 adds support for GCC
6. Add an GCC 6.x option for external toolchains, too.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/toolchain-external/Config.in

@@ -768,6 +768,10 @@ choice
 	  Set to the gcc version that is used by your external
 	  toolchain.
 
+config BR2_TOOLCHAIN_EXTERNAL_GCC_6
+	bool "6.x"
+	select BR2_TOOLCHAIN_GCC_AT_LEAST_6
+
 config BR2_TOOLCHAIN_EXTERNAL_GCC_5
 	bool "5.x"
 	select BR2_TOOLCHAIN_GCC_AT_LEAST_5