sanity.bbclass: Detect and fail if 'inherit' is used in conf file

sanity.bbclass: Detect and fail if 'inherit' is used in conf file

'inherit' directive may not be used in conf files as it's supposed
to be used for the inheritance of classes.
Correct form in conf file is INHERIT.

This commit adds:
- a sanity check to find whether the wrong case exists
- fail the build if so
- tell user about the difference in directives

[YOCTO #5426]

Signed-off-by: Gregor Zatko <gzatko@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>