build_tools: Fix download_externals to allow the use of curl or wget
Not sure if this is really a bug versus an improvement. I can see it being
viewed as a bug though by some.
The current build_tools/download_externals file depends on wget in order to
download external modules. The current build system is able to discover
which tool to use for fetching remote files - either wget or curl.
This patch takes advantage of this capability by modifying the two calls to
the wget binary to instead use what was discovered by the build system.