Accept kernel source dir(s) specified by ./configure

This adds ability to set the location of the kernel via defines
when building from the spec files. This is useful when building
against a kernel installed in a non-standard location.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1874