The default bootstrap Go (Go 1.4) can only be compiled on a limited
number of platforms compared to newer versions of Go.
This adds a config option to use an external bootstrap Go, e.g.
installed through the build system's package manager or downloaded from
golang.org.
See: https://github.com/openwrt/packages/issues/11731
Signed-off-by: Jeffery To <jeffery.to@gmail.com>