libc/kernel: Add rules to autogenerate device specific kernel headers

This change will automatically post-process kernel headers
specified by device, board, and product. This will allow us
to not check in each kernel header twice, at least for the
device specific headers for now.

Change-Id: I3bb144b6535504b7c26b807daa75de495554356d
Signed-off-by: Dima Zavin <dima@android.com>