Makefile: fix shell error for darwin major/minor version check

Fix shell error:
 /bin/sh: line 0: [: too many arguments

for the darwin major/minor version check.

It seems for os_x_before for some reason DARWIN_MAJOR_VERSION and
DARWIN_MINOR_VERSION are empty. To fix this set DARWIN_MAJOR_VERSION
and DARWIN_MINOR_VERSION to be evaluated once so the value is retained.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>