toolchain-scripts.bbclass: Allow sourcing of subscript for environment

toolchain-scripts.bbclass: Allow sourcing of subscript for environment

Sometimes we require extra environment settings to be available on the
environment for proper SDK work. This were done, in past, using
'_append' tasks however with the split of the environment in a
canadian package this has been broken.

The easier and more flexible solution is to use environment subscripts
which are sources by the main script. These are now looked at:

 $OECORE_NATIVE_SYSROOT/environment-setup.d/*.sh

and sourced.

(From OE-Core rev: 457291f2ca084d1f43c0cca2175b448a22761887)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>