makefile: remove make export <all variables> instances

Replace all kernel make export <all variables> instances with
export of specific variables.  Depending on existing build
environment, exporting all as environment variables and
interaction with existing environment variable expansions
can lead to undefined behavior.

Change-Id: Ia46600be605828f9e509d5ced5d3ad3f8c6cb140
Signed-off-by: David Ng <dave@codeaurora.org>