oe-setup-build: Change how we get the SHELL value

There are times that a user might not have SHELL set for some reason.
We should default back to a known shell in the event that SHELL is not
set.

Signed-off-by: Ryan Eatmon <reatmon@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>