meta/classes-recipe/toolchain-scripts.bbclass: eliminate bash-ism

meta/classes-recipe/toolchain-scripts.bbclass: eliminate bash-ism

Sourcing a script with arguments is a non-standard bash extension
and doesn't work with other shells (e.g. dash, which is used on
Debian and derivatives). This used to work by coincidence when
running against integrated poky repo without having to separately
specify where bitbake is, but no longer does.

Using set is a POSIX standard.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>