kernel: allow subtarget specific KernelPackage

Add a call to KernelPackage/$(1)/$(BOARD)/$(SUBTARGET) to the
KernelPackage macro. This allows to add kernel packages for x86/64,
without breaking x86. It's not possible to do this with BOARD, as
BOARD=x86 for x86_64.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>