Speed up download-toolchain-sources.sh when /tmp is in different filesystem

Use $NDK_TMPDIR instead of hard-coded /tmp/ndk-$USER.  By default
NDK_TMPDIR is /tmp/ndk-$USER/tmp/build-$$, but developers can
define it to be at the same fs as the specified <src-dir>

Change-Id: I7b270906152694099ce43157e5f8839ac656ceee
1 file changed