multilib: Move redefinition of STAGING_DIR_KERNEL

multilib: Move redefinition of STAGING_DIR_KERNEL

If the STAGING_DIR_KERNEL is set in the multilib.conf, then it may be
set incorrected.  The evaluation happens before TMPDIR and LIBC are
defined in other components.

Moving the definition process to the multilib.bbclass ensures that
everything has been loaded before it is set.

(From OE-Core rev: 6bd87edc383b40e300b0ef4bf851c39b698305cd)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>