linux: hide custom patches when using a local tree

When using a custom local tree, we're using the OVERRIDE_SRCDIR
internally, which means we do not apply patches. Since this is the
expected behavior, make BR2_LINUX_KERNEL_PATCH and
BR2_LINUX_KERNEL_CUSTOM_LOCAL options exclusive.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>