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>
diff --git a/Makefile b/Makefile
index 7eaf454..fa4f38b 100644
--- a/Makefile
+++ b/Makefile
@@ -318,8 +318,8 @@
 #
 ifeq ($(HOSTOS),darwin)
 # get major and minor product version (e.g. '10' and '6' for Snow Leopard)
-DARWIN_MAJOR_VERSION	= $(shell sw_vers -productVersion | cut -f 1 -d '.')
-DARWIN_MINOR_VERSION	= $(shell sw_vers -productVersion | cut -f 2 -d '.')
+DARWIN_MAJOR_VERSION	:= $(shell sw_vers -productVersion | cut -f 1 -d '.')
+DARWIN_MINOR_VERSION	:= $(shell sw_vers -productVersion | cut -f 2 -d '.')
 
 os_x_before	= $(shell if [ $(DARWIN_MAJOR_VERSION) -le $(1) -a \
 	$(DARWIN_MINOR_VERSION) -le $(2) ] ; then echo "$(3)"; else echo "$(4)"; fi ;)