sanity.bbclass: Add check for @ character in build directory name

The @ character is not escaped properly in too many places within the system
to easily support it so add the character to the list of blacklisted characters.

Also tweak the other messages and ensure that all appropriate error messages
are disabled in one go.

[YOCTO #4772]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
sanity.bbclass: Fix COREBASE sanity tests

We need to expand the COREBASE variable, no idea how these tests were
previously working at all...

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>