techpack: makefile: do not export all the variables

This patch fixes an issue with all the functions defined in
scripts/Makefile.lib gets called everytime when this makefile is
included thus causing the build to slow down.

Bug: 118818737
Change-Id: I4929e64d53a0bc865b12a27cce37520be52c9744
Signed-off-by: Thierry Strudel <tstrudel@google.com>