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.

ASTERISK-26812 #close

Change-Id: If9411a2554f009274d377445613ae91192d948a1

build_tools/download_externals

@@ -103,7 +103,7 @@ version_convert() {
 	echo ${v}
 }
 
-${WGET} -q -O ${tmpdir}/${variant_manifest} ${remote_url}/${variant_manifest} || {
+${DOWNLOAD_TO_STDOUT} ${remote_url}/${variant_manifest} > ${tmpdir}/${variant_manifest} || {
 	echo "${full_name}: Unable to fetch ${remote_url}/${variant_manifest}"
 	exit 1
 }
@@ -188,7 +188,7 @@ fi
 
 if [[ ${need_download} = 1 ]] ; then
 	echo "${full_name}: Downloading ${remote_url}/${tarball}"
-	${WGET} -q -O ${cache_dir}/${tarball} ${remote_url}/${tarball} || {
+	${DOWNLOAD_TO_STDOUT} ${remote_url}/${tarball} > ${cache_dir}/${tarball} || {
 		echo "${full_name}: Unable to fetch ${remote_url}/${tarball}"
 		exit 1
 	}