core: allow product to provide kernel headers as well

This is useful when a device inherits from an architecture
product, such as omap4.mk. The architecture specific product
can provide kernel headers to all the devices that use
that architecture.

Change-Id: I0c9d3120a6089fb535a3fa8eccf10775a271cd21
Signed-off-by: Dima Zavin <dima@android.com>