buildtools: Handle generic environment setup injection

Currently buildtools has its own hacked environment setup. We added
capability for generic script fragements but this code was never updated
to use it even if several components it contains needs it.

Remove a hardcoded variable in favour of the generic scripts to
avoid bugs in this area in future. The others should probably be
migrated to environment files in future too but there isn't such
a file for those at present.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>