kernel: don't assign the build user/host

kernel: don't assign the build user/host

The KBUILD_BUILD_USER and KBUILD_BUILD_HOST variables were
assigned at the kernel class level, which made it impossible
to override them in the local configuration.

By setting only the default values of those variables in the
kernel class, it is now possible to override them as expected.

(From OE-Core rev: a3e8cdf9c3ba966fa4b5a21235540eb0b00fb487)

Signed-off-by: Federico Sauter <federico.sauter@ableton.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>