populate_sdk_ext.bbclass: Make integrated buildtools optional

populate_sdk_ext.bbclass: Make integrated buildtools optional

If the host system is expected to have enough capabilities that the
buildtools-tarball is not required, we don't need to bundle it.

This can save some significant space, especially when using with a minimal
eSDK.

minimal eSDK - core-image-minimal-qemux86-64

with buildtools-tarball     - 34 MB installer - 281 MB installed
without buildtoools-tarball - 11 MB installer -  48 MB installed

Signed-off-by: Mark Hatle <mark.hatle@kernel.crashing.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>