kernel-source do not exist in STAGING_KERNEL_DIR any longer.
Remove the exported KERNEL_SOURCE as the STAGING_KERNEL_DIR can
be used in module recipes directly.
(From OE-Core rev: b67ef579db4c1906d6e508a8451cd2b01b7b62ab)
Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>