linux: add KERNEL_ARCH_PATH to fix x86-specific issue

Both i386 and x86_64 architectures are supported by the arch/x86
directory in the kernel. So, when we copy the kernel configuration
file to arch/$(KERNEL_ARCH)/configs/, it does not work because
arch/i386 and arch/x86_64 do not exist.

So, we introduce KERNEL_ARCH_PATH, which is the path to the
architecture specific directory in the kernel source tree.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>