Use ext-toolchain.sh to wrap external toolchain commands,
abort build if certain features such as CONFIG_SOFT_FLOAT or
CONFIG_IPV6 are enabled but not supported by the toolchain.
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@29766 3c298f89-4303-0410-b956-a3cf2f4a3e73
