kernel-yocto.bbclass: Fixup do_kernel_configcheck usage of KMETA

kernel-yocto.bbclass: Fixup do_kernel_configcheck usage of KMETA

The do_kernel_configcheck task requires a meta directory, normally
set by ${KMETA}. The meta directory is taken as a relative path
from ${S}:

        outfile = "{}/{}/cfg/mismatch.txt".format( s, kmeta )

However, when checking for the presence of ${KMETA} the current
working directory is searched. This will almost always fail and
"kgit --meta" is used instead. If the user does have a path in
their current working directory that matches the ${KMETA}
variable but the path is not present within the kernel source
directory, the build will fail if it tries to write config errors/
warnings to that path.

If ${KMETA} is not set, the same problem exists with the hard-coded
"meta" directory.

Fix these issues by checking for ${KMETA} within ${S} rather than
the current working directory. Additionally, drop the hardcoded
backup directory "meta" as it hasn't been functioning and
probably has no users

Signed-off-by: Russ Dill <russ.dill@nikolamotor.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>