toolchain-external: make preinstall the default for custom toolchains

All the defconfig files used by the autobuilders that use
pre-installed external toolchains are making the assumption that the
default for a custom external toolchain is "pre-installed". We keep
this default for now, since changing it breaks the autobuilders.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/toolchain-external/Config.in

@@ -573,6 +573,10 @@ endchoice
 
 choice
 	prompt "Toolchain origin"
+	# Keep compatibility with old defconfig files that are using
+	# custom toolchains, and which are therefore assuming that
+	# "preinstalled" in the default choice.
+	default BR2_TOOLCHAIN_EXTERNAL_PREINSTALLED if BR2_TOOLCHAIN_EXTERNAL_CUSTOM
 
 config BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD
 	bool "Toolchain to be downloaded and installed"