populate_sdk_ext: get rid of buildtools

populate_sdk_ext: get rid of buildtools

If we do `bitbake buildtools-tarball' and then after one day do `bitbake
core-image-minimal -c populate_sdk_ext', we would meet errors like below.

| install: cannot stat '/buildarea2/chenqi/poky/build-systemd/tmp/deploy/sdk/
poky-glibc-x86_64-buildtools-tarball-core2-64-buildtools-nativesdk-standalone
-1.8+snapshot-20150429.sh': No such file or directory

The problem is that the output name for buildtools-tarball has ${DATE} in it.
So if populate_sdk_ext task is executed but buildtools-tarball is not rebuilt,
the above error appears.

In fact, ext SDK does not have to depend on buildtools. This patch removes the
buildtools from the ext SDK. After this patch, the ext SDK still works and we
can no longer see the buildtools error.

[YOCTO #7674]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>