multilib.bbclass: fix faulty redefinition of STAGING_KERNEL_DIR

multilib.bbclass: fix faulty redefinition of STAGING_KERNEL_DIR

Due to the problem fixed in
'56c677a multilib: Move redefinition of STAGING_DIR_KERNEL'
STAGING_KERNEL_DIR must be redefined for lib32 in multilib.bbclass.
However this redefinition expanded STAGING_KERNEL_DIR to an absolute
path. This unconsciously added the TMPDIR path in the sstate object,
causing packages depended on STAGING_KERNEL_DIR being rebuild if the
TMPDIR was changed.

Solve this by forcing the unexpanded TMPDIR variable to remain in the
beginning of STAGING_DIR_KERNEL (as default). Since TMPDIR is included in
BB_HASHBASE_WHITELIST, the sstate object will not be depended on the
expanded path anymore.

Signed-off-by: Petter Mabäcker <petter@technux.se>
Signed-off-by: Ross Burton <ross.burton@intel.com>