sanity.bbclass: sanity check for if bitbake is present in PATH

If a user executes the environment script instead of sourcing it,
there's an error about an empty element in PATH. This is because
bitbake isn't present in environment variable PATH. Hence, this
patch adds a sanity check to verify if bitbake is present in
PATH and if bitbake isn't present issue a warning message.

[YOCTO #13822]

Signed-off-by: Dorinda Bassey <dorindabassey@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>