toolchain/toolchain-external: warn for untested GCC/kernel version

The oldest toolchain we test in the autobuilders is the Sourcery ARM
toolchain which is GCC 4.8 and kernel headers 3.13. Therefore, it is
likely that we're missing the required _AT_LEAST dependencies to exclude
packages that don't build with older GCC/headers.

Add a comment to the custom external toolchain that warns when an
untested GCC or kernel headers version is selected.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

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

@@ -92,6 +92,9 @@ config BR2_TOOLCHAIN_EXTERNAL_GCC_OLD
 
 endchoice
 
+comment "GCC older than 4.8 is not tested by Buildroot. Use at your own risk."
+	depends on !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
+
 choice
 	bool "External toolchain kernel headers series"
 	default BR2_TOOLCHAIN_EXTERNAL_HEADERS_REALLY_OLD
@@ -298,6 +301,9 @@ config BR2_TOOLCHAIN_EXTERNAL_HEADERS_REALLY_OLD
 
 endchoice
 
+comment "Kernel headers older than 3.13 is not tested by Buildroot. Use at your own risk."
+	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_13
+
 choice
 	prompt "External toolchain C library"
 	default BR2_TOOLCHAIN_EXTERNAL_CUSTOM_UCLIBC