toolchain-external: add gcc 8 entry

This patch allows to use an external toolchain based on gcc 8.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

toolchain/toolchain-external/toolchain-external-custom/Config.in.options

@@ -27,6 +27,7 @@ config BR2_TOOLCHAIN_EXTERNAL_CUSTOM_PREFIX
 
 choice
 	bool "External toolchain gcc version"
+	default BR2_TOOLCHAIN_EXTERNAL_GCC_8   if BR2_ARCH_NEEDS_GCC_AT_LEAST_8
 	default BR2_TOOLCHAIN_EXTERNAL_GCC_7   if BR2_ARCH_NEEDS_GCC_AT_LEAST_7
 	default BR2_TOOLCHAIN_EXTERNAL_GCC_6   if BR2_ARCH_NEEDS_GCC_AT_LEAST_6
 	default BR2_TOOLCHAIN_EXTERNAL_GCC_5   if BR2_ARCH_NEEDS_GCC_AT_LEAST_5
@@ -37,8 +38,13 @@ choice
 	  Set to the gcc version that is used by your external
 	  toolchain.
 
+config BR2_TOOLCHAIN_EXTERNAL_GCC_8
+	bool "8.x"
+	select BR2_TOOLCHAIN_GCC_AT_LEAST_8
+
 config BR2_TOOLCHAIN_EXTERNAL_GCC_7
 	bool "7.x"
+	depends on !BR2_ARCH_NEEDS_GCC_AT_LEAST_8
 	select BR2_TOOLCHAIN_GCC_AT_LEAST_7
 
 config BR2_TOOLCHAIN_EXTERNAL_GCC_6